It seems clear enough from context that the "GPL restrictions" are that if they used the GPL-licensed compiler, the commercial clients might run into legal issues with their use of it, necessitating that they purchase the commercial license. It's not uncommon for businesses to have a prohibition against using GPL software in not only their shipping products but anywhere in their toolchain. (You can argue that's a counterproductive prohibition, but "your legal department just needs to change their mind on this" may not be an argument a vendor can effectively make.)
I would not make an argument even if I thought a client would accept it. If they are incompetent they will decide to use the GPL code with sloppy oversight, violate the terms of the GPL, then they will hold a little grudge against you for the advice that got them in trouble. Sloppy companies have no internal accountability, so it's your fault.
I use GPL code all the time at home and I would license many things GPL, but there's no reason to push GPL software at corporations. They should have limited options and spend money, possibly expanding MIT code, possibly just raising the price of engineers by keeping engineers occupied.
