Yes he does indeed say that, but nothing in that statement tells me that knows "definitively" that they were under enemy fire. They opened fire when the mine exploded because the assumption was there were enemies on the banks of the river but that doesn't mean there were indeed enemies on the banks of the river firing at them.
There were mines exploding, artillery being fired...a boat was disabled, at least one man went over board & maybe even a dog flying through the air
in short it was chaos. He could have thought they were under enemy fire when what he heard was fire from the Swift Boats.
Which brings to question: boat disabled, if they were drawing enemy fire they would have been shot to death, no one was.