Unlocking Mastery for boosted toon

So I boosted a toon. I filled the XP bar and I keep getting this message "Your current ability is fully trained and ready to master!"

I see that it won't unlock until I do either "Torn from the Sky" (whatever that is) or "Sparking the Flame" (from Path of Fire).

So am I supposed to skip all the Living Stories before that and jump directly to it? Or am I supposed to go through it all in order and live with that obnoxious message all the time (and lose out on all the xp I'm not getting credit for since my xp bar is full)?

Well you skipped levelling, so jumping ahead to do one story mission, that is repeatable at a more relative time, shouldn’t be a big deal, no?

This is the result of a conflict of interest between players. Some people absolutely hate the idea of wasting XP (even in a game with infinite XP) and want to prioritise unlocking new abilities for their character over anything else, and are happy to play little bits of the story out of order to do that. The game is set up to allow them to do that and the prompt makes sure they don't miss it. (Especially because the mastery system is a bit different to the process of unlocking skills and traits, and didn't exist in the game before HoT so even for veteran players it was a new concept.)

In theory people who want to prioritise playing the story in order can do that too, but for whatever reason there's no way to get rid of that message. (My best guess is whoever put it in is that first type of player and didn't consider that some people would want to delay unlocking masteries.)

If you want to get rid of it the only way is to play the first step of either the Heart of Thorns or Path of Fire story. Both involve spoilers for their respective storylines but you may be able to avoid those by skipping/ignoring cut scenes and not reading or listening to NPC dialogue. Or you can ask someone else to run through it while you're afk to avoid them completely.

Completing Torn from the Sky (part 1 of HoT) will unlock glider basics, allowing you to glide anywhere in Tyria. Completing Sparking the Flame (part 1 of PoF) will unlock the raptor mount, which can also be used anywhere. So you might want to consider which of those is most likely to be useful. (Personally I recommend the glider - the raptor is mainly useful as a speed boost whereas the ability to glide can save you a lot of hassle if you fall.)

Is he your only character? I just boosted a new character and he had access to gliding and mounts already - because my account did.
If it is your first character I would go unlock gliding and mounts right away. The gliding helps if you fall, and the mount helps you travel faster. Both worth getting right away.