Awesome article Gareth.
I am personally a big fan of the Design Sprint and I could see first hand the benefit of doing guerilla user testing at the early stage of the projects. Though, having been an “agency” designer myself for years, I can also understand the point of view of your UX Team.
From their perspective, design by committee -or involving other testers / stakeholders early on can be a real danger.
I still have in mind this infamous comic from the Oatmeal “how a webdesign goes straight to hell.”

The great thing with the Sprint is that you get to show your ideas and test them after five days, and this gives you just enough of time to put all your best ideas together, polish them a bit and prototype your “best shot”.

The designers nor the team won’t get frustrated because they did the best they could within that timebox. And the user’s feedback will be invaluable.