It’s been a pretty common practice for years for online services. At my previous job we implemented something like this too, making a guess at the gender of users based on just their name and some additional information. The only reason we did that is because we didn’t want to ask people for their gender when they registered, we only needed the data for investor reporting and marketing decisions, and both of those use cases are OK with a rather large margin of error.

I assume Discord would do this for similar reasons.