Who said, "all sensors are temperature sensors, but some are better than others"?
Whoever said it probably did so in person, and the odds that any of us met the true first person are low. This is the sort of engineering observation that could have been invented by several different people back in 1940, for all we know.

Elecia White's 2011 Making Embedded Systems quotes this anonymously, so the latest possibility is 2010. Her PowerPoint also has:


  All sensors are temperature sensors, some measure other things as well.


An alternative, suggesting some divergence:


  It has been said that all sensors are temperature sensors first.


This follows the broader observation that no real-world physical relationship or behavior is truly, fully linear across the range of all possibility. For electronic sensors, temperature is big, but humidity, pressure and acceleration pop up surprisingly often. You could say that the best sensor for one phenomenon is just exceptionally bad at sensing all the others - because anything that interacts with it does something.