Yes, if there's no large-scale test data on humans we have no definitive answer if it works.

I didn't know about the article you posted. The original link I posted was data from late 2020 (released this month) when no one even knew Omicron existed so it's unclear if that formulation works for Omicron. The phase 3 data from ARCoV still hasn't been published to my knowledge, and your article says at the end ARCoV-Omicron is approaching clinical trials, so there also isn't any data there either.

Note also in the phase 1 ARCoV trial they didn't even try it on anyone over the age of 60, which should be the target population.
Developing a mRNA vaccine is nontrivial especially in China because they can't license the IP from other companies and have to do everything from scratch. It's not similar to an inactivated vaccine like Sinovac where the technology is well known.