One of the other millennium problems is counting to 7.
Not really. There's a non-pedantic, charitable interpretation widely available. It's below for reference.
> Navier-Stokes is one of six [open] “Millennium Problems” on a list compiled by the Clay Mathematics Institute in 2000.
If solved problems don’t count anymore, the sentence also doesn’t make sense because now navier stokes apparently isn’t open anymore too, right?
Maybe? But then the parent comment doesn’t make sense either. (There are 2 solved.) Like I said, take the charitable view. To a lot of readers this will be “announcing” the closure of the second of 7 problems so is internally consistent.