The rationale to consider them part of the main Marvel universe is the same than that for the "Tales of the Watcher". They're related by a 616 character, so the comics are 616. But I think the argument is too flimsy for these titles, and looks like and excuse to bloat the Marvel universe and make it more encompassing than it originally was.

Uatu is a much more important character than Digger. It also talks about things it has seen, so things which supposedly take place in 616.

Digger is a marginal character which initially had no connection with the Marvel universe. You could say that neither did Hank Pym, but they changed their mind about this one pretty quickly, while Digger wasn't used for anything else in over a decade. Also, the stories narrated in his mag need not be anything else than that, stories. The case for the stories hosted by the other character (Headstone P. Gravely) is hopeless, as the character hasn't even been featured in any other Marvel comic.

There are more cases:

This one with Jim Hammond and Toro, would have a right to be part of the MU, but being originally done in the 50's (although not published until '68) is more like a GA comic (and just as bad) and more of a Timely production.

This one supposedly features Odin, but it doesn't look like Ditko was in touch with Kirby about his version. Hardly the same character.

Another one by Ditko, with the mythical character Merlin, a very confusing case, and it doesn't help that it's supposed to be the GA character or that it comes from a world not yet created then.

Quogg was shamelessly used only once, 45 years later, just to say it was 616. Again, Ditko was just doing a monsters comic.

Going back to myth, Medusa appeared twice in the early 60's, but it's unclear they're the same character.

And last but not least, the epitome of retcons. When Byrne (who else) went back to Ditko to rescue the "first mutant" for The Hidden Years. No one has gone back to that in over 20 years, although you have to admit that Steve beat Kirby on this one. He just wasn't doing a superhero comic, that's all.