Can I keep track of one specific app's sales from someone else's iTunes Connect page?
They are correct, a user on iTunes Connect has access to all apps, no matter what their role. Obviously this is a terrible design, but Apple has never been too quick to fix design flaws in iTunes Connect.

As you mentioned appFigures has the ability to restrict a sub-user's access to individual apps. The publisher will have to set up the primary account.

I can't find official information from any of the other analytics providers that they have this capability. My App Annie account definitely does not.