It wasn’t viable at the time, makes sense they’d focus on exploring other avenues.
from the very same source you provided:
The broadest and perhaps most important conclusion from the MSRE experience was that a molten salt fueled reactor concept was viable. It ran for considerable periods of time, yielding valuable information, and maintenance was accomplished safely and without excessive delay.
It wasn’t viable at the time, makes sense they’d focus on exploring other avenues.
Remains to be seen if it’s viable today as well mind you.
from the very same source you provided:
Viable as in theory it worked.
Not viable as in this makes economic sense to continue with in comparison to other avenues.