Does that run on all the BSDs and nicely on low-performance embedded ARM boards, which are explicitly targeted by NTPsec? Does it talk to low-level OS APIs for time management without complicated wrappers?
It's a replacement for one of the many background UNIX services, not a "primary" service that can own a machine.
They run something called Java Card, which is a very limited subset of Java (missing things like threads, enums or floating point numbers). And they are highly standardized, so you can be sure it is not just available and supported, but optimized for running these applets.
That doesn't really say anything about viability for an open-source project that's supposed to run on all kinds of embedded and general-purpose hardware.
It's a replacement for one of the many background UNIX services, not a "primary" service that can own a machine.