Good website copywriting is honest, even when it means comparing yourself to the rest. If you’re going to compare your company, product or service to the competition, you need to be honest. Naturally you want to mention the differences that put you ahead of the competition, but don’t short change the competition or make up negatives they really don’t have—that tells the reader they can’t trust what you have to say.
Source: expresswriters.com, 11 Website Copywriting Secrets (That We Will Tell), #10
Shortchange or short-change are more usual ways of writing this today.
It means, literally, to give someone less change than they are due: for instance, if you tender a $1 bill for an item costing $0.59, and I give you only $0.31 in change, I have shortchanged you by $0.10.
But the word is more commonly used in a figurative sense, as it is here: the author is telling you to speak honestly about the actual value which your competitors provide and not give them less credit or respect than they are due.
See for example Collins.