I had that same question and the sales guy did not know.
I thought they locked in the down position.
Personally, I thought it was weird as my 98 Chev, the seat bottom locks up & down and the back locks up (makes sense) but not down.
I have the seat backs down in the Chevy to create a table like platform that is easier to access from the front seats. Since this is a 2dr ext cab most of the access is from between the front seats.
I haven't locked mine yet but I believe the way the folding lockable storage under the seat works is that you lock the seat down (using a key though). So if you have the feature you can lock down (key) and lock up (latch).
Obviously I don't have the truck anymore but I could swear my 2017 seats did not "lock" up because I don't ever remember pulling a latch release. I could reach back from the driver seat and fold down the rear set. Did it several times when I had to pick my girls or their friends up from school.