That is really the heart of it. And a worthy objection, in my opinion.
If this were some new rule, I would also be appalled.
But the essence here is to prevent abuses. The Church has used grape wine and wheat flour since the time of the Apostles, for deeply biblical reasons.
All we are saying is that commercial "gluten-free" products are not assured of keeping biblical purity. Let's make sure that we stay with what we were given from the Apostles as we strive to accommodate those with modern allergy problems.
This letter offers guidance toward how we can do both -- preserve the integrity of the gospel sacraments and the health of believers. It is pastoral care when we need it, which is why Jesus gave us a Church.