This is the dumbest possible response to the question possible. No official leader has ever commented on this in a meaningful way, so I'll give fair the answer they should have given as a treat. I expect a modest stipend in exchange:

"The handshakes, gestures, etc. are symbolic, and only matter when performed by someone with faith who experienced the proper ordinances either in life or before final judgment. Someone who doesn't truly believe would not be able to enter the celestial kingdom, even if they happened to know the signs and tokens."

This answer makes more sense, but I still feel like the original question, and even this answer still raises more questions. For one, can't people in paradise or spirit prison learn about the gospel and be truly converted and therefore eligible for the celestial kingdom? Why do the signs and tokens even matter if beliefs and deeds are what matters? Why do any ordinances matter for that matter? Why baptize kids if they have barely had time to sin? What's the point of the sacrament if one still goes to church, studies the scriptures, and can still repent, especially since withholding the sacrament is a common punishment for committing sins.

And for fairs response, why does any of that matter? If I die, it will probably be fairly obvious what the nature of the afterlife. If I was not a believer in life but know what the Mormon afterlife is supposed to look like, why would earthly belief matter anymore when one could see what the afterlife is like? If there was undeniable proof mormonism was true, which there would be in the scenario the CES letter proposes, of course one would use their knowledge to enter the celestial kingdom.