Imagine a terrible day: you go to your oncologist to hear about the programme of chemotherapy they've worked out for you. The lights flicker in the office, the doctor is wearing Crocs, he has a t-shirt under his medical coat, which has coffee stains on a cuff and ink stains on a pocket. He has an impressive number of successful remissions.
Imagine a tedioius day: you go to your company's internal communications session. Someone asks if there's any chance they could dress down, or have dress-down Friday. The CEO says 'no, I don't want to hear any more of this, we're a professional company.'
Who are the professionals? The scruffy doctors who save lives, or the employees of a software company with three rounds of redundancy under its belt since the recession and twenty-year a track record of not growing?
This is a tasteless comparison. But this is about more than the right to wear jeans. Doctors don't grade each others' professionalism based on how crisp the collars on their coats are. They do it based on performance, dedication to quality medical service, good bed-side manner, publications, results, results, results. But in the world of software, which aspires to professional status, people regularly use 'professional' to mean smartly dressed. This is a problem. Do they really have such a frightfully superficial definition of what constitues professionalism? Do their customers know?
Actually, they're probably just making a category error. Dress is not something that falls into the category of 'professionalism', it's something that falls into the category of 'corporate culture'. What they mean is 'we're a corporate company.' But this sounds like a weak and pernickety excuse, so they don't say that. (I don't see why, because companies have culture, and corporate culture is a real thing and that should be fine.)
So, either the proponent who makes the category mistake is dim-witted enough to believe that dress is part of professionalism, or they're deliberately using poor argument in order to win and shut up the whining wage-slave. In the first case they're probably intellectually mediocre, which isn't great for big, high-stakes projects. In the second they're actually not dedicated to truth, which itself is a form of unprofessionalism.