Starlin has more than once written Thanos as failing/losing. Like you said, the Thor loss was not bad and had heavy context to it. And like you said, the Ka-Zar fight is misrepresented itself. Ka-Zar does not defeat Thanos in battle. He just performs better than he should have in one scene.
I believe that Starlin mostly wanted to keep continuity in line with his previous theme of Thanos no longer being a nihilist. He actually retconned an Avengers issue whereas Thanos doesn't lose and has high power showings. But he was a nihilist in that issue.
Starlin basically retcons two "losses" and does both in one book on one page. Yet some will make it out as if he has individually retconned double digit losses.
As far as Slott, his whole things was basically making fun of the retcons.
In Starlin lore, Thanos really only loses because he wants to lose for one reason or another.
There have been more retcons than the ones indicated.