Why does YouTube disable the comment button until text has been entered?

YouTube disables the comment button by default, and only enables it once text has been entered:

Before: enter image description here

After: enter image description here

The general consensus on ux.stackexchange.com is to not disable submit buttons and wait to present errors until after the user has hit the submit button.

Does this mean YouTube is going against best UX practices? What would be the reason for this?