The same way WATA (may) have done it (in my opinion), or anyone with enough capital could do it (as there is no pop report). Refer investigation by Karl for details.
There might be a policy that says something like 'CGC employees can't sell or buy CGC graded cards'...well WATA does too, and it appears that's what WATA was engaging in at its highest levels for graded video games (rules just being a front).
The reason why this works is because there is a lack of information with regard to rarity (WATA has no pop report, just like CGC). Hiding the pop report is not effective for cards that are fleshed out (ie. All the cards that collectors were grading with PSA for years, which are a different thing). Like I said before, low pop (new or nobody graded because it wasn't rare). Those cards, believe it or not, are selling for very high amounts in 10s/perfect 10s. I would argue that they sell for more than PSA cards (specifically the cards without a fleshed out pop report in CGC 10s).
I came back into the pokemon hobby about 2 years ago and since then I've heard about trimmed cards by PSA (allegedly), trimmed cards by psa being knowingly sold by PWCC (allegedly, as I believe the FBI is or was investigating this), PWCC being accused of shill bidding by eBay, and Karls investigation into WATA directly manipulating the graded video game market in conjunction with heritage auctions (no pop report being instrumental for this to occur). So when I see a company that is as well organised as CGC, primed right at the hype of the popkemon boom, not release pop report (but have no issues with AI, QR codes to online data bases) for no real reason...well...its concerning
But at the end of the day, maybe im just an insufficiently_thoughtful_person who doesn't know the first thing about market manipulation.