MOCs are allowed to Prioritize their slate of 10 nominations. (Which I personally prefer, because I believe it's THEIR responsibility to choose their best nominee). If your MOC does list a "Principal" nominee, then that person is automatically guaranteed an appointment; so long as s/he is qualified.
Now, maybe your MOC gave you a "Principal" nomination. If so, then you pretty much have a guarantee of an appointment. On the other hand, they could have used a poor choice of words. It's also possible that your senator put in a "Competitive" slate of names, and the academy already chose you as the appointee and contacted the senator. Again, we don't know.
So, to answer your question, yes it is possible that you have a guarantee. But until I saw that guarantee in writing, I would continue on with everything as though the senator never contacted you. Wait till you see the BFE (Big fat envelop) or at least a change in your online application. Best of luck to you. Mike....