One other thing to consider here, is that there are two situations you
are discussing which involve "shall not impede" ... a narrow channel and
a TSS.
There can be differences to both. In the case of the narrow channel,
there's a good chance that the ship is limited to to the confines of the
channel, for whatever maneuvers it can or may attempt.
However, this does not always apply to TSS's. In a TSS, there's a good
chance that the ship can maneuver fairly freely to avoid (which may and
can take them outside the TSS), so that if push comes to shove, they can
maneuver as in Rule 8 (which is why the writers of the rules have not
made the "stand on " condition, absolute, as Shen has stated).
If you take SF Bay, there are a number of TSS areas where some ships
could physically go outside the "channel" ..... I would not expect them
to and don't doubt that unless an emergency they won't, as VTS will have
a fit, but again the legal aspects would take over in case of collision.
I fully agree this is an area that is confusing. My opinion FWIW,
consider yourself (the small boater) as the one required to give way,
and ignore the possible legal stand on ramifications which would come up
in court.
The most important issue, is to avoid the collision.

otn

ps At one time when TSS areas where coming into being, this was a USCG
"gotcha" question. "You are in a TSS and see a small power driven vessel
crossing your bow from stbd to port on a collision course. Which vessel
is stand on and which must give way?"
The answers were set up so that it was obvious that even though the
small boat was not to impede your passage, you were still obligated to
maneuver, since you were the giveway vessel in a crossing situation.