Note that Crawford is speaking generally. Not specifically about the Forgotten Realms. 'Some' paladins serve a god or a group of gods. Paladins from the Forgotten Realms would fall into this category.
And again, I point to mr Greenwood's comments. The Oath is a pact made in the name of a deity the serve. Piss off that deity and don't expect to keep your powers.
You are misrepresenting how polytheism works in the realms. A paladin of Helm might say a prayer to Umberlee before boarding a ship. That doesn't make them a paladin of umberlee because umberlee isn't their divine patron. Except in a select few circumstances such as Masked Traitors (Vhaeraun/Lolth) or the Triadic Knights (Tyr/Ilmater/Torm) you don't see multiple gods empowering the same individual. If you are a Paladin of Helm, that's it.
