I wouldn't read that much into it without even asking for elaboration, "it fell apart" can mean a lot of things including "they messed it up after I left". That it really just fell apart under its own weight without anyone doing anything is probably the weakest plausible interpretation of what they wrote. After all, if it brought in $100M/year yet was of low quality, why didn't they hire some people to recreate a better version of the thing real quick?

Well, if he's the sole maintainer and they don't replace him, falling apart is inevitable. Software dies when its dependencies do and that doesn't imply low quality.

That depends on why it fell apart, and what he would have done about it.

Here is one possibility. The developer checked on the lead system every day, which mostly ran itself. After the developer left, they checked a few times and then stopped looking at the lead system's reports because it was always good. Then months later another team replaced a key system that the lead system needed. Nobody noticed until sales reports sucked. By the time they fond and fixed it, they lost $X million and the people who got fired got fired for not having followed the recommendation to keep the system monitored.

I've seen variations of that story before. And I've seen companies get hurt because of it.

Now there might be blame for the developer. But there are enough ways that the developer wouldn't deserve blame that I wouldn't assume the worst.