It has worked for years without setting an official rule, let's keep it this way.
The discussion linked by jlliagre in a comment first took place 9 years ago, at the very beginning of this site, and it seems like the biggest preference was for French as the main language on this site. While I do agree that as a learner, forcing yourself to speak the language you're learning helps you making progress faster, I do not agree with setting this as a rule, native or not.
Personally, I want this site to be as welcoming as possible for learners. I have witnessed it around me: not everyone learns the same way, and some people are not confident enough (or their level is not advanced enough) to ask a question in French. In the same spirit, it seems well-intentioned to ask some questions in English so that potential learners who stumble upon them are able to understand them and make progress.
Also, I'm a native French speaker and yet the only time I ever asked a question (on Meta) was in English, because it felt more natural on SE and also more inclusive. Maybe that's how the OP felt on the post you linked.
So my opinion is that, native or not, we let people use the language they are the most comfortable with at a given time, maybe we encourage them towards French in a nice way like you did, and everything should be fine! :)