I tend to agree on principal, but funnily enough I had a bad knee issue happen to me on my first fully loaded trip, with reg pedals and toe straps.

I honestly figure it was a combination of a lot of factors-- I know I was overloaded and undertrained; there were steep steep buggers of hills that probably overtaxed my already overworked knees; I may have had bad technique when standing (maybe twisting a bit side to side) and working like a mad fiend getting up a real bugger of a hill (toe clips totally tightened, so no "float" at all); I had Biopace chainrings that could have been a factor too; and to top it off, I had pushed my seat forward to reduce the reach which I learned afterwards is bad for knees.....all I know is that my knee swelled up and I had to stay at a campground for a few days before continuing.

A year later when I got spds , I was concerned about knee issues with spd pedals so paid for a fit to be done at a good store, where they had a frame setup to take all your measurements, and also pedals that showed your natural knee angle on each side--it showed my right foot turns out naturally, so they adjusted the cleat so my foot was in its natural position and it has been fine since then. I have essentially copied that cleat position all these years now.
As mentioned, certain pedal systems have more float and are most likely easier on knees, but yes I agree, if setup badly, a clipless system can harm a knee more if someone isnt aware of cleat positioning at all and or ignores messages from ones knees.