In short, price A/B testing is legal and common practice for most companies, and VWO's article seems a bit too quick to jump to declaring it illegal.
[1] https://en.wikipedia.org/wiki/Sherman_Antitrust_Act_of_1890
[2] https://www.law.cornell.edu/wex/Clayton_Antitrust_Act
[3] https://en.wikipedia.org/wiki/Robinson%E2%80%93Patman_Act
[4] https://www.ftc.gov/tips-advice/competition-guidance/guide-a...
[5] https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A...
In particular, if a customer might have seen multiple prices for the same thing around the time of purchase, for example on a pricing page checked on a phone and then on a checkout process completed on a PC, you would surely want to have solid evidence that the actual price charged had been clearly understood and accepted at the time of payment.
[0] https://www.econstor.eu/bitstream/10419/205221/1/de-Streel-J...