Worth keeping the reporting bias in mind in this case. It's extremely more likely that a home user will send data to popcon than a mainframe which may not even have a full internet connection. I wouldn't be surprised if many are run in isolated networks and with local repos. (I don't expect it to be more than 10 either way of course)

I doubt many mainframes are running Debian.

IBM recommends Red Hat, SUSE or Ubuntu. All three come with commercial support contracts. If you are already paying big bucks for a mainframe, it seems far more likely you'd go with one of the IBM-recommended distributions than go with Debian.

(Ubuntu may have an interest in keeping Debian on mainframe alive because doing so helps them in offering Ubuntu on mainframe.)

And I'd doubt even more that they're running Gentoo -- especially on older hardware, where having to compile everything from source is even more of a burden than usual.