Have you considered that maybe their code is just bad?
If your app is open source, I say just build & test on one of the major distros and let the community port it to others. If its closed source, well, good luck. But if what the parent said is true, that you now collect a bunch of very vocal pissed off customers because you didn't support their favorite distro, then its just not worth it at the current marketshare that desktop Linux has.
There's also the challenge of you just can't make any assumptions about what may be present or not on someone's Linux machine, even with the major distros.
I personally think they should port it, but, the developer product does already run on Linux, in the terminal, as is the case with the majority of other dev tools.
The comment was that the Electron apps aren’t being released for Linux even when they exist because Linux is so much harder to support, even in Electron.
If they don’t have resources (or desire) to keep the Electron app working on all the Linux distros then they definitely won’t have the resources to write a completely separate GTK app for the few Linux users.