Straight from Brann when you ask him “How do i earn progress towards the Delver’s Journey?”
His answer is “Openin’ a Heavy Trunk, Bountiful Coffer or Hidden Trove at the end of ANY delve will earn ye progress to access the rewards we’ve found.”
He says “any” delve not just bountiful. This is not whats happening.
I fail to see your point. I’d assume non-bountiful delves would give less coins and progress, and lower levels would too. You should be able to progress in this solo content how you want. If someone wants to grind out some tier 4 non-bountiful delves because it’s what is fun for them, why does that concern you? Go do your t8+ delves and don’t worry about it.