Quite possible the Level 3 mounts were allowed to be ridden at lvl 20 just for Beta testing. I'm simply sharing my experience with what happened IN GAME to me. I'm thinking they will implement it as you say, we will be able to ride our Spiders as a Level 1 mount until it is trained to higher levels (+speed and dismount).…
I bought an Armored Worg (level3 mount) and it was considerably faster than normal horses and was rideable at lvl 20. I'm assuming the Ranks determine how difficult it is to be knocked off he mount, NOT the speed.