Update: it seems the known issues list was incorrect regarding the Slitherdrake and it is, in fact, time gated with the next campaign chatpers coming next week, as Blizzard clarify:
Yes! Chapters 4 and 5 of the questline will arrive next week, and that will include the Slitherdrake mount.
We also have some updates to in-game text that should clarify this coming soon.
Looks like Warcraft players will have to wait a little longer for the new Slitherdrake Dragonriding mount to become available, as an issue is still preventing access to Chapter 5 of Embers of Neltharion campaign. Blizzard is currently working on a hotfix and has pushed a list of known Patch 10.1 issues.
Known Issues (Source)
Welcome back, adventurers! We hope you are once again ready to fight alongside your friends for this world. While this list is not all-encompassing, below you will find frequently reported issues currently being investigated: