For anyone interested in why it was renamed from Riot, znpy posted a comment under a now heavily downvoted comment below[1] that links to an explanation blog post[2].
Honestly I think “riot” is pretty recognisable as a software company and it’s fair that they defend their brand against another software company. You wouldn’t expect to be able to trademark “epic.im” or “blizzard.im”.
Different area of business (games vs chat program). But I see your point, all companies you mentioned have a messenger with mobile apps (AFAIK).