Why would anyone use iMessage/SMS on a corporate/work phone, if the corporate messenger is already installed?

Because that OS is far too old/EOL? Nobody bothers writing software for ancient platforms like Android 5, and why would they? Just so like 0.000001% of their userbase could use it. It doesn’t make any sense for them to do it.

So I guess you’re posting this from the Ukrainian front lines, right?

No.

Because most of those people are idiots.