I personally like to know whether the software I use/endorse is developed with broad inclusion in mind. It is a pretty solid indicator of thoughtfully-designed software.
A developer assuming, for example (not related to this instance), ~all the users are able-bodied men with perfect vision and motor skills, leads to an inferior product that is guaranteed to alienate people who don't match the "target audience", and those people already experience a life full of being ignored or not getting the same degree of accommodations the majority do.
Using gendered language is just a small facet of the full spectrum of ways that people are excluded (even if it's just to a small degree), and it's nearly effortless to think "yeah, it would be good to use wording that makes sense for any reader of this text", and is simply a kind thing to do. When a developer actively fights against this, I can only imagine their motive is exclusionary and prejudiced, or perhaps just ignorant.
It's disappointing my post was downvoted to oblivion but I'm not surprised. If someone doesn't care about using inclusive language, that's not really a reason to suppress someone raising a concern about it. What is especially surprising/disappointing is that it's flagged, despite being a totally valid "PSA" about the exclusionary stance of the software's founder (at the time of the linked PR -- which I already disclaimed in the OP). The post violates no rules/guidelines and adds value to the conversation.
Regardless of all this, to any dev reading and thinking "why do I care", I would ask, why are you developing software? Fundamentally, it's to provide some value and use to people, right? That value/purpose shouldn't come with the caveat of making someone feel excluded or overlooked. Or, if you only care about providing that value to a certain set of people, at least make that very clear up-front so the rest of us can know to skip over and look for something else.