That's good, but I don't see how that means we know they aren't being stored. The NSA or another entity could be collecting them in bulk just in case the encryption is broken in 30 years.
And sure enough, even today the most practical attacks on DES are in effect brute force on those too-small keys and too-short blocks - this brute force is practical although very expensive.
Now, in 2001 DES was superseded by AES, which is used for this purpose by Signal. In AES the keys are larger (128, 192, or 256 bits, Signal uses 256) and the block size is longer too (128 bits), thus fixing the only problem DES still had. We are done.
So, while of course nobody can prove it won't happen in 30 years, it is extremely unlikely.
You might think to yourself, surely computers get faster and smaller, so the brute force problem would still arise maybe in a few decades? Nope. In the late 20th and early 21st century humans experienced something that's only going to happen once, and it has distorted our perspective on the matter. The machines are not, in fact, going to keep getting faster and smaller, there are physical limits imposed by the universe. They will get gradually a bit faster than they are now, and we will make a lot more of them, but nowhere close to enough even if (for some insane reason) our civilisation decided its only goal is to decrypt my old Signal messages.
Anything that goes over a network could be stored.
"user messages are stored only on their devices, not on Signal’s servers or anywhere else."
Whether or not a 3rd party, outside Signal's knowledge and/or control, is storing messages is entirely out of their control.
Suppose I promise you that qMsVOrgWDZTo0Fet9xLhIQ is the base 64 encoded, 16 byte encrypted message I just sent, but I used OTP. Do you in some sense "have" a copy of my message ? No. That is completely useless without the key, it could have literally been any message, without the key there's no difference.