At least it sounds as if this is just an issue of generic rate limiting, not specific to Mastodon.
In that sense, this is fake news, as it falsely suggests Twitter has special rules in place to shut out Mastodon.
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there's a little star on the documentation saying that an app can only create 300 Tweets per user or per app for every 3 hours. I believe that the influx caused a lot of new power users from twitter, that post a lot, to come over and that's now causing the crossposter to hit the limit.
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They're not overly inquisitive, but it seemed that a human judged the comment you write, and there was some delay, so it's probably not something most people would do if they want to give a third party service access.
The form you fill is only needed for upgraded access, it requires filling multiple pages with minimum character limits (not very Twitter like). It was approved automatically for me, but the basic API is fine for regular users. If you do any kind of analysis though, basic and upgraded Twitter APIs are useless and too rate limited even if it seems generous.
For example, I have a colleague who has been a big twitter user. They’re a passive listener on the site, and want to move to mastodon because people are moving. They created an account on mastodon.social and struggled to find their people. I suggested some tools that would help… and they refused on grounds that they were afraid the tool would abuse their twitter login information.
I wound up trying to explain oauth scopes. They didn’t get it and still haven’t tried. There is no way this person would sign up for a twitter dev account. But they are on Mastodon and putting some effort into figuring it out.
That said, the proportion of people making the move who also want to set up a crossposter might well be much more technically inclined.
some power users are hitting the 300 post/3hours limit according the linked thread.