Hence the "unless/until". I'm not "handing" anybody anything for the week-1 roster, that will be revealed 5 months from now.
For now, though, Waddle & Cannon are my "default" starting OTs since they have extensive NFL starting experience and extensive experience in the Pats' system. Tobin's NFL starting experience (and tested athleticism for me) puts just below those two, but his lack of Pats' experience make him "third".
Croston has a bit of live-fire NFL experience and a year of healthy practice in the Pats' system, so he's also a "candidate". However, Garcia and Jelks have nearly zero experience and both have been away from meaningful snaps for a considerable time.
Basing a current, temporary, penciled-in depth chart exclusively on experience is no more (or less) arbitrary than any other method. However, the relative experience of the players currently on the roster is a fact. At this point, anything else is really just subjective evaluation and speculation based on things that only the coaches can possibly know for certain at this point (and even they may not be certain quite yet).
IOW, Waddle is a "placeholder" unless/until someone else produces evidence - that I can see - that they've stepped up and overtaken him.