the Ramban clearly concludes that Rashi saying Yaacov lo mes means he is spiritually alive.
No. He brings the מאמר חז”ל from Rashi. Then he gives his own interpretation of Chazal. His pirush has nothing to do with Rashi’s pirush specifically, and definitely not with Rashi on Shas (which is what we are discussing).
Honestly, I don’t think we even know which way the Ramban held, because his words here can be understood both ways (as opposed to Maharsha, Rashba, etc. who clearly hold that Yaakov is alive spiritually. And Rashi, Rif, etc. who clearly hold that Yaakov is alive physically).
Either way, the point is that your question is clearly answered, no matter what the Ramban held.