The baptism here immediately followed the receiving of the Holy Spirit. In Philippi, Paul and Silas were put into the jail because they freed a girl from an evil spirit. A miraculous earthquake freed them, but they remained in jail. And they spake unto him the word of the Lord, and to all that were in his house. It is very likely no water would have been available for the baptism except for pouring.
History appears to support pouring.
Lenski wrote about Rogers looking for clues in early church areas that showed immersion was used. Moreover, the remains of all the ancient baptisteries were shallow, generally so small a man could not be immersed even if he lay down in the pool flat on his back. If we examine these texts in context, we find Paul is writing about a spiritual death, burial, resurrection, and new life. He is not writing about the mode of water baptism. Such newness of life was not brought about by water.