Interesting concept. This would work for the moment, until the providers hear about this type of detection and change the session sizes or intervals. I am unsure what the intent of this type of detection would bring other than knowing that someone is using OpenVPN. Doing this at scale would be even more difficult.
There are quite a few work arounds which would defeat this testing methodology.
Since they only tested OpenVPN, and with Wireguard becoming a bigger player, (additionally the Tailscale/Headscale's of the world), this detection would never work.
I am surprised they didn't attempt to try this with Tor since those people are more likely to be more serious about their anonymity and privacy.