Understand that the certification they’re offering is not for a Microsoft-branded version of Linux but Linux on Azure in general. They have full end-to-end support for Linux on the Azure cloud platform now, something they didn’t have when they launched. It could be RHEL, Ubuntu, FreeBSD, CentOS, Fedora... doesn’t matter. Linux is Linux at its core.
FreeBSD is not Linux, BTW.
And maintaining/admin Linux is quite different across some distros. That is why some software will run under Ubuntu, but not RHEL, will run under Fedora, but not Debian.
My RHCE does not transfer seamlessly to Ubuntu.