If Yodell is Hermione.
She might have thought Sierius is trapped. Hence wont speak. And she can out claim dementor.
The dementors dont know each other, and if Yodell is a baddy, both dementors might think she is the second dementor.
Even if she knew Sirius can speak, Sirieus have no advantage to say that he had not targetted Yodell. Siriues outing himself would be dangerous for him.
So when Yodell claim to be dementor,
As a baddy, it is a very safe claim. Almost natural for baddies to go for it.