SSK
Longhaul services tend to have much lower yields than shorthaul
I don't think this sentence is correct. Yields are usually way much higher on longhaul than on short haul which is quite logical. I think you probably mixed up yield with revenue per ask, or yield per ask eventually.
I think you'll find SSK is far too knowledgeable to have mixed this up - it's just that he is using yield to refer to revenue per passenger kilometre (what is sometimes also referred to as "unit yield" or RASK or RASM).
You'll see from his definitions that he defines yield as revenue/traffic, where traffic is measured in passenger-km.
And it's quite correct to say that the revenue per passenger kilometre is much lower for a long-haul flight than a short-haul one.
(Of course the average yield
per passenger is usually higher on a long-haul flight, but that wasn't what he was saying.)