Biden could approach and pay respects, but WAS NOT allowed to 'lay' the wreathe
For comparison, here are past presidents, actually elected and recognized by the US military as their commander in chief. Wreathe is presented for the POTUS to lay, rather than already there to begin with.
Here's Trump
And Obama
And Bush
And Clinton
And Reagan
Only Biden was denied the laying. Could approach and pay respects, but not lay it. Aligns with being a foreign dignitary rather than the POTUS.
Could it be that protocol is different for inauguration vs. Tomb of the unknown soldier.
Don't have any reason to believe that.
While there's no dates for the earlier presidents, and some of the later ones appear mid term, Trump's video is from immediately before his inauguration on the 19th of Jan on 2017. If there was ever a case that it would be different in one case or another, we have spanned the gamut yet in ALL cases it's the same ... except with Biden.
Here's one of Trump where he doesn't present, but he arrived 30 minutes late to the memorial day ceremony, ie one being held whether he showed up or not, rather than inaugural ones where you're the whole reason for it (Like with the other Trump video, and Biden's non laying case).
Hell, I even found one of Prince Harry being allowed to lay a wreathe, so it tanks the 'foreign dignitaries can't lay', but it's still a case of Biden not being permitted when you'd think he would.
Literally the only example we have of not, are when you show up late to a ceremony going on whether you're there or not, and Biden.
Harry laid a tiny wreath. We let him, probably it is because UK was an allied fighting WW2.
I made a similar assumption.
While it seems any citizen can apply to lay a wreath, I found no video in searches for foreign dignitaries, ambassadors, or heads of state doing so. But admittedly, those may not have been video recorded or uploaded. Even finding presidential video was not easy. YT only wants to show you Biden and Trump.