It should go without saying -- but quite obviously cannot -- that no one can be 'President-elect' until they have actually been elected President, and that in the United States nobody can be elected President until the Electoral College elects them. The Electoral College meets on the Monday after the second Wednesday in December, which this year is December 14. Indeed, arguably no one is formally President-elect until Congress counts the Electoral College votes and declares the results, which occurs on January 6, although unless there are controversial slates this is just the final stamp. There is currently no President-elect; Biden is projected by unofficial sources, based on still-being-finalized official information, to become President-elect, which is significant but not the same thing. There would be no point in saying this, and people might be allowed their sloppiness, except that in a republic the actual process of the election matters, and it continues to matter regardless of how much anyone might want to short-circuit it.
This pet peeve of mine is particularly irritated right now because the GSA recently opened the transition process for the Biden campaign. This does not represent any kind of status change for Biden, nor does it represent any acknowledgement of Biden as President-elect; it is purely an administrative procedure by which Biden gets access to certain funding and administrative resources that are set aside to facilitate transitions. But of course there were reporters who were shocked, shocked, that the letter starting the process didn't refer to Biden as 'President-elect'. It's not surprising that it wouldn't -- 'President-elect' is not an official title of the United States, and there is in any case no President-elect yet, so there's no requirement that it would have to do so -- but such is the genius of some reporters that if an entirely routine administrative process takes place they can invent a scandal out of it.