Here is an explanation of what was possible when a Debian packager mistakenly introduced a patch which reduced the SSL certificate keyspace in 2008: https://jblevins.org/log/ssh-vulnkey
The possibilities of keys which were generated by this random number generator was so small, that a brute-force attack on keys was feasible.
That being said, for years, random number generators have been using random signals coming to your computer (key strokes, network packets, ...) and feeding them into a sponge function. You don't need lava lamps or pendulums to generate random numbers, it's just for the press.
If implemented in full tool assisted speedruns, this can become entirely ridiculous. That rare thing you need to grind hours to find? nope. just press these 18 buttons starting at the right frame and it'll drop first try.
If you have something that's geared towards speed running, fast execution and mastering, being deterministic is actually a good thing. Otherwise, the player has to re-roll a hundred times to get the "good seed" to get a good run - that's very frustrating. Look at some of the kings quest speedruns for this kinda frustration.
Or in a similar direction - tetris with a true random piece selection can result in very frustrating and unwinnable piece sequences (which bastard tetris is dialing to 12). Instead, quite a few modern implementations choose some set of tetrimonios and fill a bag with 3-5 repetitions of this set and choose from that. This limits the possibility and length of possibly frustrating pieces.
At a GDQ it's fun to watch some awful exploit "win" Mario in eight seconds (not a real example) by messing with a game bug and precise input, but it's also fun to watch somebody who is incredibly good beat it the way you'd imagine you could if you were much better at the game, so there's room for multiple categories in popular games.
If there's a split it can often be resolved by having two categories. If you think it's stupid to hope to get a good seed in Minecraft, you can just only run the chosen seed categories, meanwhile if you think that sucks because it doesn't reward agile thinking you might prefer the random only categories. Both groups get to have fun.
Each has to be secure with a unique unpredictable element.
One way (of many) is to use complex PRNG's - Psuedo Random Number Generators, but these can (theorectically) be sequence guessed and then are predictable (or least have a greatly reduced "guess space").
A hybrid way is to regularly reseed a PRNG with a "truly random" starting point, milk that for a few hundred values, and then kick it again with a new random seed.
These particular pendulums have "simple" mechanics that are similar to the dynamic systems represented by Lorenz's Butterfly [1] (not exactly but with similar properties) and thus move in chaotic unpredicable ways (such as the timing between the last crossbar changing directions) - these can be "driven" (the base rocked | vibrated) by a small motor) so they continuusly move and yet the path through phase space is still unpredictable.
So, without knowing for certain whether this is what Cloudflare is doing here, that is one way in which many many truly random numbers can be generated to feed into secure handshakes; using unpredictable seeds to feed into twisted PRNGs.
The same could be achieved electronically with a simple circuit inside their servers, but it wouldn't look quite as good.
Also, somewhere in the thread, they mention that it is probably too hard to verify how it is actually implemented in the hardware.
https://www.reddit.com/r/crypto/comments/klcx4p/is_true_rand...
I guess in security only the paranoid prevails...
Or it is indeed just a cool gimmick...
If you can guess how the random key is generated, then you can decrypt the message without breaking either encryption algorithm. That's why it is important to have really unpredictable random data, and physical randomness is the best for that.
In practice, that wall of pendulums is unlikely to make a difference compared to the more mundane techniques used in most computers these days, that rely on some CSPRNG combined with various hard to predict system events (ex: interrupt timing), but it looks way cooler.
But it is also a source of entropy that is hard to manipulate (if we ignore the cameras, image generation etc). And it is a good source, initiator for discussions about entropy, random numbers and why we need them to secure things. This thread on HN shows that it is so. And for 'normal' people probably even more.
I'm sure they have a lot of non-geeky visitors that look at the installation, read the sign and start asking questions. And if it was me, starting to jump in front of the installation, waving my arms - to provide my bit of entropy to the pool.