I agree. Davis intended to do what he did all along.
If Watterson deviated and sold the merchandise he would have lacked integrity, sure. But someone like Davis is acting according to their ideals and intentions and so on; there is a lot of integrity present in the matter.
People use it to mean “they did something I disagree with” which doesn’t inherently overlap with integrity concerns.
I suppose you could argue that art isn’t meant to be commercialized in such a way, and so Davis lacked integrity to the discipline and trade. Some comic artists might say his approach takes away from comic artistry and lacks integrity in that manner. I’m not sure if that’s true or reasonable to say at all. Someone can make a comic for any reason they want as far as I’m concerned.
I don’t know what Davis is like personally, but from the outside and looking only at Garfield, it doesn’t appear that he lacked integrity.
I think I'm taking it a step further even and asking that if Watterson decided that he would never merchandise Calvin and Hobbes and then 10 years later saw things differently and changed his mind, would that be an integrity violation? I don't think so.
No, arguably not. People change their minds over time. We learn things, change, our environment changes. I'm not who I was 10 years ago, but not for a lack of integrity.