wrong
The online law is a PDF of a scan, so I couldn't copy-n-paste the relevant text. See definition #3, "large retailer", in section 3. It's pretty modest and in no way limits the law to WalMart: $1 billion in sales for the entire retail chain, 75,000 square feet of floor space in DC. Wallet and footprint definition. Reportedly six other retailers are whining about the law and threatening not to expand into DC (seems kind of theoretical, doesn't it?), so that makes six other retailers who see the law as applying to them.
In the course of researching this, I did come across the usual rightwing echo chamber parroting the claim that the law targets WalMart. You seem to have mistaken shrill propaganda for fact.