GM signed a deal with Andretti to be their exclusive supplier, but the engine won't be ready until 2028 (F1 engines are quite complicated), so Andretti was going to be a customer of one of the existing engine suppliers in the interim. They were trying to get on the grid next year so they could take advantage of the $200 million anti-dilution entry fee based on the current concord agreement, but the existing teams think this is not enough to cover the smaller pieces of the pie that would come from having 11 teams on the grid vs. 10. They just came up with the excuse that Andretti wouldn't be competitive from the get go. Well Duh! that is always the case. The just want to delay Andretti's entry so they can continue to maximize their income with a much larger entry fee in the new concord agreement.
Andretti argued that with their history and pedigree, that they would bring a lot more to F1 by growing the overall pie with the additional interest from US fans, not to mention that they already have a long history of racing in IndyCar, but FOM isn't buying that argument, so they said no, for now.
Let the lawsuits begin.