That's in the vicinity of the gear position sensor and oil leaks travel as you go down the road. I'm not saying that they're wrong, but I've heard enough dealership stories to know that they're also not always right. I would remove the starter cover and clean that entire area super thoroughly. Start the bike and see if you see the leak. If not, take it for a short ride and then inspect the area. It's most likely the gear position sensor.
From what I'm seeing in the manual, see Resources top of this page, (pg 1399 - 1403) if it actually IS the starter it's another o' ring. Not a big deal to fix, but yeah, yeah, you shouldn't have to. Rather than wait for parts and sitting at the dealer forever, I'd fix either of those issues myself. Faster and done correctly.
Btw, the Triumph service manual does not call for replacing any bolts on the starter (or the gear position sensor). What are they talking about? And, when the manual does specify that it's always because they have Loctite on them, which you can clean and reapply yourself. But in this case it doesn't even call for that.
Trust me, either of these are a very easy fix. You just need o'rings. I have a bag of the ones for the gear position sensor and I'd be glad to send you a few....