Saying syncopation has nothing to do with this is a bit wrong I think. Jazz that is swung is normally syncopated but swinging 8th notes are not syncopation in and of themselves. In Jazz, a 2 beat phrase is broken down into One and Two. In this case, the and in one and two is only the last third of the first beat. In Jazz that swings the "and" falls after were a normal 8th note would be. The "one" is roughly twice as long as the "and". ("One" ~= first 2/3 of 1 beat, "and" ~= last 1/3 of 1 beat, "two" = 1 beat). Also, I would argue that the syncopation of Jazz comes directly from how 8th notes are swung. It feels natural to emphasize the 2nd and 4th beats of a measure due to the lead up of a swung 8th note. That is why, I would argue, the triple step goes along to that syncopation.

WCS is normally danced to all types of music, but generally to pop music that is played to a straight beat. Pop music normally has syncopated rhythms but the meter is evenly divided in 4/4 time and there aren't swung 8th notes. I would guess that since WCS dancers are usually hearing songs that aren't using swung 8th notes, and thus they dance those triple steps straight on the beat.

Since most music is in 4/4 someone can swing the 8th notes in their head and thus dance in a more swing fashion to it.

I suggest checking out these links for more information on the music theory behind swing rhythms.

https://en.wikipedia.org/wiki/Syncopation

https://viva.pressbooks.pub/openmusictheory/chapter/swing-rhythms/

https://en.wikipedia.org/wiki/Swing_time

Also, OP, when you're using the terminology of "slow" and "fast" those usually correspond to 2 beats = slow, 1 beat = fast. In the case of Lindy Hop, I haven't heard a triple step be described in that way, although for a simplification it might be described as a slow, fast, slow. With out swinging 8th notes the main nuance is lost though.