Personally I would not use pi's for this. Massive waste of money, the experience is worth it but other than that..
If you don't need 99.9999 availability of your cluster and are only interested in firing up the cluster from time to time then those same 600$ can buy you a few years of cluster time
8 of those instances is equivalent to an 8 core (16 thread) chip with 64GB of memory. Using, e.g., Supermicro AMD EPYC 3251 + parts, you could build an entire server for less than 1/2 that price. You could have twice the compute for the same price the first year. Servers easily last 3 years minimum, so Hetzner has a 6x markup at the low end.
Admittedly, building your own server doesn't include transit, but transit is cheap. Assuming you have a fast home connection (a big if for many people), the hard part is access to static IP addresses. There are dedicated services that lease IPs over a VPN tunnel, or you can setup your own VPN into, e.g., Hetzner or Vultr using a handful of the cheapest instances. (Not sure about Hetzner, but Vultr let's you lease and attach 2 additional IPs to the same instance for $2/month/IPv4 address.)
Now, the above setup is probably far too much trouble for most people, and it has many downsides[1]. But the point is that if we're just talking about CPU time, not even Hetzner or some other cheap VPS provider can come close to the cost of running your own equipment.
[1] Power and cooling are two downsides, but the 3251 has a TDP of 50W. Granted, it doesn't have the highest clock speeds, but there's a reason cloud providers don't normally advertise Ghz. Anyhow, rather than build 2x 3251's you could build a single 16 core rig using a higher-clocked and newer EPYC or Xeon for about the same money (definitely at or under 2200€ all-in).
Your choice of comparisons suggest you completely missed the whole point. If the author wanted a desktop I'm sure he would buy one for 600$ instead of spending that money to put together an underperforming cluster that's entirely unusable as a desktop.
If instead you are interested in gaining experience managing a cluster that runs distributed systems then yes you would do something like putting together a raspberry pi cluster.
Yet, if you seek to have the same experience (or arguably far better) without spendig a hefty sum as capex and having access to more computational resources for a negligible opex then cloud providers such as Hetzner are clearly a far better option.
https://www.servethehome.com/introducing-project-tinyminimic...
All in that was around 450 eur over a year ago, all bought in EU.
All three running linux with no load consume around 30W.
I think the author mentioned only spending $449.73USD (i can't verify because the sites down), which seems cheaper than what you mentioned.
I'm also struggling to find exactly what you're talking about. I'm seeing 9020's, but no M, do they actively sell it still?
I'm not nitpicking your comment - i too am interested in throwing a couple Pis in a closet for some distributed systems projects i'm working on. I expect them to be slow, low bandwidth, but simple over wifi/etc. I don't want to pay more than i need for closet computers, though, so i'm interested. :)
For something cheaper, I can find in my country lenovo tiny m72, for something like 40 eur. No SSD/Ram, you can configure it as you wish (and still will be cheaper then RPi, even after adding like 2GB of ddr3 - it's something like 4EUR per stick, there are 2 slots in each machine).
That is with Intel Core i3-2120T, so would expect much more performance then even RPi4
Edit: I see now that the same seller has them for ~72eur with 120GB of ssd and 4GB of ram if someone wants something ready to use.