180 mph taken from a bit of googling, ballpark figure on upper end.
So this was really immediately after takeoff. My understanding of commercial airliners is they usually fly fairly parallel with the ground just after takeoff to pick up speed before ascending, so I would guess they hadn’t much altitude at all.
Anyway it’s a very interesting article, ty to poster! And it was an interesting question to think about.
One notable example: https://news.alaskaair.com/alaska-airlines/flying-fish/