Pretty much, but 135.1a5 is an exception. It brings a little of
part 135 (just the drug-testing) to bear on sightseeing flights,
irrespective of part 119.

Do you have any evidence that this is true? I don't buy it. Flight
instructors often take people for sight seeing tours, which is
permitted in Part 119. What sort of free lance flight instructor
would have a alcohol testing policy? Doesn't make sense.

If this were truly the case, the 119 exemption should be removed, and
135.1a5 would limit applicability to the drug testing stuff, and the
result would be the same.