The prop 65 warning is everywhere, to the point that it loses its effectiveness 

Anexample.  The collection storage rooms at a museum where I volunteer were recently remodeled and  prop 65 warning sign promptly appear on the door.  to the walls Why?  Apparently it had to do with the paint applied, which was quite dry an cured, with no hint of remaining volatiles.

I imagine thee is something similar with the trainer you are contemplating.  i wouldn't worry about it.  Just don't sprinkle the paint chips on your Wheaties.

You will probably be much healthier using the bike trainer regularly, even with the dreaded prop 65 warning.