That's a pretty big oversimplification of their argument and antitrust law usually isn't so cut and dried.
What they're saying is that in the market for product A, if sales of product A are usually bundled with product B, then the courts view product A and B as one product sold together. The fact that product B might be sold separately in other markets doesn't change the facts and circumstances as they pertain to product A's market.
The precedent they cite in Rick-Mik Enters., Inc. v. Equilon Enters. LLC demonstrates an example of this, where credit card processing services are typically bundled with sales of gas station franchises, and therefore in the market of "gas station franchise sales", "credit card processing services" were not considered a separate product even though you can obviously purchase "credit card processing services" separately in other markets.
While I don't know if this particular argument will hold up in court, it's not "so lousy & so easily proven false" either.