It does if applied incorrectly
Well then the same can be said for mylar rolls, and also could be said that you will get hot spots if your paint isnt totally flat either... You're reaching, and failing.
Or maybe it applies if the tin foil is hanging all over the place and starts a fire...
Think of it like this:
Lets say your light puts out a total of 20000 lumens.
Ok, if mylar is 95% reflective and totally flat(giving max reflection), that means a max of (20000 x .95)=19000 lumens can be reflected back. MORE lumens/energy CANT be reflected back than is original produced...
Ok, tin foil is only about 80-85% reflective, so you do the math there.
If your reflective material is incapable of reflecting back 100%+ of the lumens/energy, then how in the hell is less lumens/energy going to hurt a plant?
Also, how many hoods or reflective shields have you seen without tons of dimples, like a golf ball?
On the other hand, a magnifying glass increases energy by concentrating it, and it is only possible via the shape of the lens. Even with increasing the energy in a small concentrated spot, the overall energy is decreased. There will be a 4% loss due to Fresnel reflection off the surface of the magnifying glass.
Common sense goes along way; some people just refuse to use it in obvious situations.
If you read this, please, in the future, don't keep spreading a fucking lie.