Beam size and material for 15 feet long swing
I would use the 4X8. If I had not seen the recommendation that you have posted, that is what I would have suggested.

The round pipe is not suited for the support of weight. "I beams" and such are. The round pipe where the swing will be attached has a high potential the colapse.
The round pipe can work if a truss configuration would be made of it, which would require welding.