The commenter is referring to what's called a "secondary strike". These are heavily regulated under current law (https://www.nlrb.gov/about-nlrb/rights-we-protect/the-law/se...) and the PRO act would repeal these rules.

A secondary strike is only one kind of action, and "march on" does not clearly indicate a secondary strike although it may include that. Bannering or leafletting and other types of protest could fall under "march on", could they not?

It's legal right now, even as clarified in the link you provided, to encourage a boycott of a struck product at a retailer. What's not legal is organizing a boycott of a whole store because they are stocking one or a few products from a supplier whose products are struck. That is, unless it's a company store that is the same employer as the producer of the products for which production is under a strike.