With your finger, tap and drag the timeline indicator toward the right, but don't drag it all the way to the end. Then, release your finger, and you can click the "Skip" button.
For ads that can be skipped after a certain amount of time, videos appear to go into an everlasting loading loop if you drag the timeline indicator all the way to the right. You have to stop short of the ad timeline's end.
Then, you can click the "Skip Ad" button on the video.
The Reddit user appears to face an unskippable ad, and simply tapped toward the end of the ad's timeline on their Touch Bar to resume their video.
You can check out the Reddit user's video showing how they skip an unskippable YouTube ad.
Essentially, the user simply taps at the end of the ad's timeline, which skips the ad and lets the regular video resume.
Just remember, this is all technically a workaround, so YouTube — or Apple — could disable this ability at any given time.