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.
Lack of data is an issue. And it's largely the fact that I have Obama's 21 gun 3+1@3sec salute arriving at Arlington that I feel the claim is emboldened. These things have reasons. In the military it's x or it's y because of z, but very rarely is anything ever, you know, x or y, whatever you feel.
Not only is Biden is the only 2+1@9sec, not only does it align with the foreign dignitary, but it's literally the sole odd man out of literally every living POTUS salute on video record I can find. Granted most of those are inaugural, but it's not a low number.
The only thing giving me pause is the 2+1 of Regan's funeral. That also feels out of place, but it's not as if I have any other example of it, such as a 2+1@5sec for the death of some foreign queen or something to better align it with something other than what it otherwise would seem to be.
Lacking evidence of POTUS' receiving 2+1@9seconds is not evidence that no such occurrences exist, but the data does fit the hypothesis and despite now two days of work looking I can't for the life of me falsify it. So while I will not present it as fact (outside normal optimism/hyperbole) like all things in science I hang my hat on it until disproven or a better fitting alternative is proposed.