I see there are several online sources still offering tickets for Thursday; the cheapest reseller ripoff I saw was $84 on mlb.tickets, for one seat on the roof of Ballpark Village.

. With online scalpers, "sellout" is a relative term, with bots buying up thousands of tickets, then gouging individuals for 2 or 4 tickets with usurious prices.
Of course, in the current Cardinals zeitgeist, a lot of those scalpers might drop prices drastically near game time or end up eating them, so a reported "sellout" (ahem) could still have vast expanses of empty seats.
When was the last time the Cards had less than a sellout for the home opener?