Is that entirely accurate, though?
The Gospel makes it clear that the Spirit has already been at work in the world before Christ’s ministry as the Power of the Holy Spirit causes the Incarnation to be. Additionally the Nicinean Creed states authoritatively that the Holy Spirit has spoken through the prophets. Also, in Jesus’s own baptism the Holy Spirit descends upon him.
Even John’s own prophecy says that Jesus is the one who will come and baptize with the Holy Spirit - and Jesus does depart from John and begin to baptize disciples in John Chapter 3.
Also, the Slaughter of the Holy Innocents in Matthew marks another occasion before the Ascension where we see Sainted martyrs - those who know the fruits of baptism.
Then there is Jesus’s baptism by blood in his Passion, instituted in Luke’s Gospel.
While the Ascension marks the end of Christ’s age on the earth and the beginning of the Holy Spirit’s era, I don’t see why it would mark the beginning of sacramental baptism as Jesus already instituted Baptism while alive.