Is advertisement is a legalised form of lying?

100% yes. Most advertisements and advertisers make totally false and tall claims, which is far from reality. And they call it creativity. It is nothing but lying and society has tacitly grown to accept it.

If the strict definition of "cheating" is taken, as is commonly defined in most countries across the world, then most advertisers and the companies would have to be put behind bars for the offence.