No, it's not the traditional service time manipulation that you would see organizations do in years past. If it was, we wouldn't even be looking at a call-up for Winn until late April 2024. Instead, it has to do with new rules regarding rookie eligibility and potential draft pick compensation.
In order to remain eligible, Winn must have occurred less than 130 PAs and 45 days on the Major League roster. This means the earliest Winn could make his debut for St. Louis is August 19th based on days on the active roster, but based on the at-bats rule, he may cross that threshold if he does come up that early.
If Winn were to retain his rookie status and win Rookie of the Year in 2024, the Cardinals would receive a draft selection following the first round in the 2025 MLB Draft. This is a huge incentive for a team like the Cardinals that draft extremely well. Since this is a lost season, there is no rush to call up Winn, so if they wait just a few weeks, they can potentially end up with that luxury.
There is also an incentive for Winn, too. If he finishes top-two in Rookie of the Year voting, he gains an extra year of service time and can hit free agency a year sooner.
It sems like this is what the Cardinals are waiting for at the moment, but there is no guarentee either way. They may decide they just want him to get as much playing time now, and call him up in the next few days, or call him up in September and just get a little bit of run before the 2024 campaign.