why is a 13" tonearm design really superior?

The main reason for the extinction of long and super long tonearms our days is because there are a lot (expensive...) turntables out there which can not mount 13" and 14" tonearms at all.
There are already more than a few which have difficulties accommodating 10" and 12" tonearms (Basis Debut Gold, Linn Sondek, most plinth-type TTs - for instance).
Furthermore - of course - increased length is always a trade-off between rigidity and effective moving mass on one side and minimized tangential error and minimized VTA change during warps and uneven areas on the records surface.

I'll see in detail that Thales at the table of an audiophile friend in northern Germany in August.
From simply looking at the principle and technical problems incorporated (1st bearing being very close to the cartridge, more moveable parts than usual, increased horizontal moving mass etc.) I am really curious whether the technical execution and fascinating idea will give improved sound regarding 12" top-flight tonearms.
The advantage in tangential error is there, but you have to pay for it with several technical drawbacks.
We'll see.

Dear Antslappy, that was a common concept of SAEC with their top-of-the-line tonearms.
The SAEC 506/30 is very similar.
However - you can get that kind of alignment with most every 12" tonearm which allows for some alternation of offset angle and overhang.
It is not an exclusive built-in-feature of the two big SAEC tonearms only.
It is a matter of concept and whether you wish to align that way.
Cheers,
D.