It’s too bad that making a customized cake involves a measure of expression, but that is not enough to avoid intermediate scrutiny, under which the Oregon law is probably valid. The situation is reasonably well explained in the court’s opinion, so I would recommend you read the opinion.
In fact, as the court pointed out, it’s speculative whether anyone who saw the finished cake would even treat it as an expression by the baker, as opposed to expression by the couple as implemented through a cake baked by some anonymous third party.
How about if the baker treats it as an expression by the baker.
Are you seriously arguing, or did the court seriously argue, that whether or not something is an expression is determined by someone other than the party creating the expression? That means we can decide that any particular statement made by its originator is not an expression, implying that the originator forfeits the right even to make the statement, because we decided it's not an expression and therefore it enjoys no Constitutional protection, until someone else buys the statement, when it becomes an expression and then enjoys First Amendment protection. The artist has no Constitutional protection for his creation since it's not an expression, but the person who bought the painting does, because it is; the songwriter didn't express anything, but the singer did. Only the people who pay the commission for the statement are entitled to Constitutional protection for the expression; First Amendment rights are for sale, they aren't inherent in our citizenship.
This argument reduces Intellectual Property to farce. I don't deny that the court argued this. I maintain that it's absurd, and yes, indefensible.
You can prove this is not indefensible by actually defending it. Please note that "the court says so" or "cite the law in Oregon" and even "intermediate scrutiny" are simply instances of the Appeal to Authority, and not a valid defense.