I can for the love of whatever is holy not understand why they put 5GbE on that thing.

Massively simplified, 2.5G is 1G sped up while 5G is 10G slowed down. It makes no sense and the market agrees. The ladder of popularity goes:

1000base-T, <long break>, 10Gbase-T, 2.5Gbase-T, <long break>, 5Gbase-T. (Depends on context ofc, 2.5G is quite popular on APs for example.)

And note a lot of 10Gbase-T hardware is not Nbase-T compatible, and there are chips that do only 1G, 2.5G and 10G - no 5G.

I guess if your design doesn't work at 10GbT you try with 5? Ugh.