Medium Size Mounts - 2024

PHB - 2024
Mounted Combat

A willing creature that is at least one size larger than a rider and that has an appropriate anatomy can serve as a mount, using the following rules.

Does this mean that humans are not allowed to ride ponies or mules (which are Medium size)?

Is there something that I am missing that specifies that it is ok for humans to ride ponies?

I'm afraid not. RAW, it's like you're saying. (EDIT: Humans can be Small, though)

Very similar wording in the 2014 PHB:

A willing creature that is at least one size larger than you and that has an appropriate anatomy can serve as a mount, using the following rules.