Their custom UI framework might be all fun and games for now, but that will probably change once they realize they need to implement accessibility. Doing this in a custom framework without sacrificing performance won't be easy and is going to require lots of messy, per-platform work. It's not like it's optional for them either. It would be for a simple editor that you can just decide not to use, but they're positioni…
Why bother with accessibility. If a disabled wants to use an IDE, use a different one??
Disability isn't a permanent state that you start with. It's something that can happen to you 5 years into your career, or 15. It can also be temporary - you break your leg and now you need crutches, a cane or a wheelchair until you heal, for example.
Accessibility also helps people who you wouldn't traditionally classify as disabled: Designing UI to be usable one-handed is obviously good for people who have one hand, but some people may be temporarily or situationally one-handed. Not just because they broke an arm and it's in the cast, but perhaps they have to hold a baby in one arm, or their other hand is holding a grocery bag, or they're lying in bed on their side.
Closed captions in multimedia software or content are obviously helpful for the deaf, but people who are in a loud nightclub or on a loud construction site could also benefit from captions, even if their ears work fine.
So, ultimately: Why should someone who's used to using a given editor have to switch any time their circumstances change? The developers of the editor could just put the effort in to begin with.