Should a submit button be disabled by default for an inline form with only one input field? [on hold]

For example:

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.

Would this mean that YouTube is going against best UX practices? Or is this justifiable behavior?