"myIRC" doesn't exist. The name of the protocol is IRC. The name of a popular Windows client is mIRC. WikiLeaks called their leak "mIRC logs," which is where this trope came from.
The United States military (not just the Army) uses Internet Relay Chat for a whole lot of C2. It runs on a network, both IRC and IP, dedicated to the purpose. Given how long IRC has been in existence, that they've been doing it since the early 90s, that the use case is the perfect ideal for IRC, and that the average modern Web app is less reliable than my last Datsun, I have a hard time finding incredulity at sticking with something that works.
Even beyond that, IRC is text-based. It is not cryptic. An IRC client is a common first software project. About twenty lines of Go gets you a bot. You can make IRC look exactly like Mattermost with a week of hacking in your favorite framework of choice, and then you're not reinventing fanout. Entire products have been built atop IRC.
Even beyond that, guess how many protocols are decades old in just the software development workflow for this product. Pretty tired of "that's old, let's do it better," even though I hate finding myself on this side of defending IRC.