I won't say it's harmless, but there are material differences. Weed is an expectorant. That's why it makes you cough. So it also tends to promote the lung functions that help clean the gunk out, at least partially. Nicotine and the additives in cigarettes on the other hand suppresses these same functions. And lastly, pot smokers don't smoke nearly the same volume as tobacco smokers.

So no, it's not harmless, but it is in fact less harmful, which is why people keep telling you that.

Also, there's plenty of other options for ingesting THC these days.