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They could pull the same trick with pay, healthcare, or anything else they offer when their employees are starting. There's a reason they don't do that.

They may not alter the existing terms but they could change future offers they make.

Requiring someone to come in to the office when it's been promised all along that he would be WFH permanently?

https://en.wikipedia.org/wiki/Constructive_dismissal

As someone who wants to stay remote, this theory gave me anxiety.

It's not really realistic. Remote workers would often have no intention to relocate. If a company changes its policy later, the majority of those people will just quit. It would also create a ton of bad will and so there is no real advantage for a company to self-immolate like that. Hiring is hard, but a mass exodus of your best employees is harder.