With LLMs it suddenly became much easier. I suspect that the timing of this decision might be not random.
I am reasonably confident that Google are more aware of what's going on with dormant accounts on their platform than you are. I previously worked at Google in Security and while it wasn't in user protection, I know they have quite a few signals for determining compromise. It's pretty trivial to see that an account that was in legitimate use for 5 years and then dormant for 5 vs one that was created and left dormant for 10.
Yes, on Google and on other platforms, account age is a valuable reputation signal but it isn't the only one.
Google deals with abuse creation accounts in the hundreds of millions.
Especially given you're talking of "thousands" of accounts, I suspect whatever problems mattered to you the most were not the same as those of Google's account system.
They mention this being mostly about security and preventing the use of hacked accounts, and they mention these dormant accounts that were only created and then went immediately dormant. It makes sense to me that they are in fact talking about this issue, just not explaining all the detail.
If the only indicator you have for spam potential is [created an acccount] + [was inactive for some period], I can't imagine the other myriad spam vectors aren't getting hammered 24/7.
I wouldn't discredit it so quickly. I have seen lots of old accounts being taken over on eBay lately for scam auctions. It seems to become more and more common. So this could very well happen with Google accounts as well. It makes them more credible for whatever scammy thing they are being used for.
Previous company I worked for had to create a process to clean up unused developer accounts. The fact that they were free to sign up meant we eventually got quite an impressive amount of them.
All this does is tell the spammers exactly how long they are going to be allowed to have 'sleeper accounts' for, the problem won't magically go away.