@slips it's totally right.

If Toyota wants to modify A/GPL code and use it on, eg, their manufacturing line robots, they're 100% allowed to do it without providing the source.

Now if Toyota wants to sell those robots... that's now a violation.

Some people seem to think that the GPL means "no private modifications can ever exist" which is not true at all.

So in this case, if it's a single user instance the only person that needs to have access to the source is the person using the software. Your ability to send and receive posts to/from this modified Mastodon server does not matter. You are not an injured party by not having access to the source code. But if you get an account on that server, you can absolutely demand the source.