snarky
critical or mocking in an indirect or sarcastic way.
I think the constructive information is mentioning Github Pages were built on .md file and they have existed for 10 years. That's true and useful.
The snarky part happens when phrasing the sentence "Ah you guys only doing this now? It has existed for 10 years" that part is talking about something constructive but it's also trying to diminish the discovery of someone else.
We all find and develop at different times. In general is better to be constructive and happy someone else is joining you in using something than underline how it was already done in some way by you or others and this new discovery is irrelevant. Everything that exists already existed (in some other shape or smaller parts).