Not a JavaScript developer, but if they can do timings fine enough to run Meltdown attacks in JS, I think they can safely and accurately measure network timings.
Timing in general isn’t the issue here, there isn’t a way to my knowledge to get just the DNS portion (specifically across an arbitrary set of DNS providers) of the network timing in JS. Just like there isn’t a way to test the timing of a single packet really either.
See original context, it is clearly about evaluating DNS providers with real end users via a web app. Considering a majority of providers don’t support it, seems it’ll be a very limited evaluation if it followed your proposal.
