Site's trying to imply that any slower 2/4 rhythm subdivided into triplets can be re-written as 6/8, whereas 3/4 does not have more rhythmic subdivisions in the three beats.
When I think of 6/8 time, I think of triplets as well. Not saying triplets don't play out in standard 4/4, but I remember thinking about how many triplets were going to be in the music whenever we were handed new music in 6/8 time. I just assumed it was me making that assumption rather than it being an actual thing.
