This is at least sufficient to show that the New Testament doesn't always use the word "baptism" to mean, literally and exclusively, "immersion in water."
Since the catecombs show believers --- and even Christ ----being baptized by having water poured over their heads, you can certainly see that from the earliest times, people were baptized by pouring as well as by immersion.
There is "one baptism" today (Ephesians 4:5). That one baptism is the baptism that saves, and that is water baptism.
Just because there was a departure from the Truth early on in Christianity doesn't make pouring a valid mode of baptism.