I've heard that argument before but I am very skeptical- there is no advertising copy on it and I don't see how it was intended as a sales piece.
I have a similar strip from '53, just sold a '48 Leaf boxing one yesterday, and have numerous Fleer 3 card strips. But none of those have any advertising copy on it and as such are just 3 card panels. One auction house recently told me that with salesman samples selling for so much these days they would call any uncut panel a salesman sample. Somehow that didn't surprise me, but it didn't make it accurate either.
Edited to add: Below is the uncut '53 Bowman sheet that I own. You'll notice all the cards are in numerical order, starting with Reese in the upper left, and then going across and then down. This was how Bowman laid out their set. It would seem to underscore my assertion that there is nothing about 3 card '53 panels that would make them salesman samples, other than the fact they are cut into 3 card panels.
