1639 does not require the government to teach the class, so if nobody created a class with the required subjects then no FFL dealer could transfer a semi rifle after July 1st. The antis would be perfectly happy with that.
If the government conditions the exercise of a right or requires a citizen to do something that depends on the voluntary participation of private industry, the refusal of private industry to participate then becomes grounds for the the law in question to be struck down. For example, whenever insurance is mandatory it is accompanied buy a prohibition on insurers refusing to cover you for just this reason.
Even California has it in their law that dealers must perform transfers, and has a maximum fee they can charge. To do otherwise would be to risk having their law struck down when someone couldn't have a transfer done for them.
Which is exactly what I said should have happened when WA passed its universal background check law, as they don't have similar provisions. All we would have had to do was find someone who lived in a small town that only had two or three FFLs within an hour drive and get them to all agree to refuse to do a transfer, and now that person has standing to sue. I sent the suggestion to SAF after their lawsuit was tossed for lack of standing, but never heard anything back.