Yeah but Smegol is a liar.

Even if his oath is strong, he's a liar and that's all it really takes for him to toss aside anything he swore too.

All he cares about is the ring, and how he's portrayed in the films, it fits as he understands that the closer the ring gets to mordor the greater the chance the ring has to go back to "Him" or be destroyed.

And also in the context of the films, Smegol already feels like he was betrayed in the first place. Not much of a reason to hold up a bargin when the other side fucked you over already, right?

Smegol is a sick twisted mentally destroyed character corrupted by the ring. No oath is going to hold him except the unspoken one between him and the ring that destroyed him.