The trick only works on Apple's Safari web browser with the 'App Controls' Touch Bar setting.
The Touch Bar settings on Macs should be set to "App Controls" by default, but you can check by going to Settings > Keyboard > and checking the "Touch Bar shows" option.
For ads that can be skipped after a certain amount of time, you'll see how long you have to wait to skip the ad, and a video timeline should appear on the Touch Bar.
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.