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It is perfectly legal for that person to decide to vote for someone else.
I believe that is actually not the case - https://en.wikipedia.org/wiki/Chiafalo_v._Washington
Chiafalo v. Washington, 591 U.S. ___ (2020), was a United States Supreme Court case on the issue of "faithless electors" in the Electoral College stemming from the 2016 United States presidential election where the Court ruled unanimously that states have the ability to enforce an elector's pledge in presidential elections.
Maine and Nebraska do this (sort of).
Also, each EV corresponds to an actual person who is sent to vote for who becomes president. It is perfectly legal for that person to decide to vote for someone else.
If Biden only manages the bare minimum 270 EVs then he's dependent on all 270 pledged voters to actually vote for him.