In case you haven't heard, CGP Grey is restricting the comments under his videos to users who have been manually approved by him. He mentions in his video about it that you need to sign up in order to be manually approved.
What he doesn't mention in the video is that you need to sign up for his Patreon. If you visit his Patreon page, you'll see that it costs upwards from $3 to sign up.
Bias
One of the purposes of the YouTube comment section is to provide a place for people to fact-check videos, which helps prevent the sharing of misinformation.
The overwhelming majority of CGP Grey's Patrons are inherently fans of him. This means that the opinions of the commenters are artificially biased in his favour, and those who are able to comment are way less likely to point out mistakes and present opposing arguments.
Since CGP Grey does not share misinformation, this isn't a huge deal. However, this sets a precedent for other less honest creators to restrict the comments under their videos in a way that lets them protect their dishonest narratives, while also making money off their Patrons who don't see through their dishonesty.
The YouTube comment section doesn't provide any hint that it's in approve-only mode, so it's hard to tell whether a certain comment section is artificially biased in the creator's favour in this way.
YouTube membership
YouTube allows its creators to set up a membership system for their channels. Users who join these memberships pay a monthly fee in exchange for benefits, just like Patreon. One of these benefits is the ability to participate in a members-only comment section, which happens to be the whole purpose of this experiment. The only downside is that YouTube takes a larger cut than Patreon.
However, this system would still allow non-members to comment in the general comment section, unlike the current system with Patreon. One can only speculate why CGP Grey isn't making use of this system.
Other methods
The current lowest price tier that CGP Grey provides is $3 per month and provides benefits other than the privilege to comment. Patreon allows for lower price tiers, so CGP Grey could add a $0.50 tier that only provides the ability to comment.
He could also use a Google Form or some other bot-resistant way to collect the names of users who should be approved. This could potentially be automated, unlike the current system.
Reddit threads
r/CGPGrey is the subreddit where CGP Grey posts links to his videos, where people on Reddit can comment and deliver criticism. Unfortunately, these comments can only be seen by people specifically navigating to these posts, which is a small percentage of his viewers.
It should be noted that the video covering the experiment doesn't have a Reddit post of its own.
Scam bots and sex bots
CGP Grey claims that the purpose of this experiment is to place a monetary barrier between bots and the comment section, which is exactly what Twitter has been doing with Twitter Blue and receiving backlash for.
However, CGP Grey hasn't mentioned attempting to use ThioJoe's tool to get rid of these scam comments. According to Linus Tech Tips, a channel with over 15 million subscribers, this tool "is so dead simple that it's shocking" and its false positive rate is "roughly zero".