This is my guess too. It's a very weird design decision, especially for a developer targeted laptop. I would probably be less unhappy with a 16 GB RAM chip soldered, but than an 8 GB one.
i get that for achieving higher memory speeds, you need to often solder instead of making it available as a slot.
but most people would like to upgrade, and if it is going to most likely work on the slower speed of the second stick, it defeats the performance point altogether.