The new decision says that the Rogers Test should only be applied to things that are not in and of themselves trademarks. So, if someone is referencing the Pepsi logo in their movie, or in an advert, or in a painting they are selling, you should apply the Rogers Test to see if that is a violation of the trademark or not.
But if the Pepsi logo (or something that Pepsi claims is too similar to it) is the only one showing up on their beverage packaging, or on the shoes they're selling, or on a T-shirt and all of its labeling, even if that T-shirt is being given away at a charity event and is thus non-commercial - well, then they are using a trademark as a trademark, and now they can't rely on the Rogers test. Instead, they have to be judged on the buyer confusion standard (would a regular person believe they are getting an official Pepsi product when seeing this item, or not?).
The SCOTUS decision is all about shutting down the short-circuit that the lower court applied, now requiring them to go back and decide on the confusion question.
The Rogers Test comes from Rogers v. Grimaldi, 875 F.2d 994 (2d Cir. 1989), which asked whether an Italian film named “Ginger and Fred” infringed a trademark of famous film stars Ginger Rogers and Fred Astaire.
The alternative is called “nominative use”, which is to use the mark of another to refer to the other’s goods or services.
For example, a cola manufacturer can write in an advert, “Better than Coca-Cola!” and it does not infringe the Coca-Cola mark, because the name is being used to identify the competitor’s product. If the same company called its own product “Better Coca-Cola”, however, that would be using the trademark as a trademark and it could potentially infringe. (Probably would, in fact.)