I think it's less to do with hipness and more to do with business realities. For example, perhaps "Monday" is an unenforceable trademark, and/or "Monday" is already a registered business entity.
It also has some mild advantages in terms of auto-generating links to their homepage when referring to the company by name in places like Teams or Slack.
.com has always been valuable. The problem is that it is very very very hard to get a short, relevant .com domain with a reasonable price tag.