The iMac Pro RAM can only be upgraded by a service technician because it doesn't have an external cover that is use-removable.
(see iMac Pro memory specifications - Apple Support)
So you are correct my friend.
Being a certified Mac hardware technician myself I would strongly go the service provider route versus the Apple Store, or at a minimum compare prices at both places. I just find the service providers much more flexible and it's always nice to have options. Many of them will even do on-site work for an extra fee. Transporting iMacs is never fun and is something that should be avoided unless you've kept the original packaging. I've known too many people who've dropped theirs trying to transport it.
iTech Gurus (very close, and they can almost certainly do it)
8475 Hickman Rd
Urbandale, IA 50322-4319
515-897-0006
If not, contact these folks (though they are a couple hours drive away)
Macheadquarters
5001 1st Ave Se Suite 106a
Cedar Rapids, IA 52402
(319) 774-2434