I think this is an easier case than the cake. The whole purpose of the shirt was speech.
Actually, if there was ever a case yet that would vindicate an owner denying custom service based upon the expressed intention for it, then Masterpiece certainly was it.
For Masterpiece Cakeshop offered the 2 sodomites any off-the-shelf items available for sale, but refused to enter into a contract to create a special work (a wedding cake, which usually must be contracted for far in advance, and typically at very high cost) which was for the express purpose of celebrating a (out-of-state) "wedding" that was/is not only contrary to the law of God, but was also against the highest law of the state at that time (the CO constitution did not recognize any homosexual marriage).
SCOTUS should have not only ruled that CO the Colorado Civil Rights Commission discriminated against Phillip's Christian faith due its antagonism, but that regardless, Phillips had a right to refuse to be complicit in the celebration of an event that was contrary to the law of His God, as well as his state.
I dont disagree that the cake case was speech. However the cake case was at least private speech. Presumably the public was not invited to the wedding. The shirt case is public speech.