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.
There was an excuse made.
Here are Covid restrictions on the Arlington National Cemetery Website, wreath laying are still taking place and nothing suggests the ceremony is modified in any way.