I recently reached lvl 54 with my PLD from 52. On that short period, I played in the dungeon and got the chance to test it repeatedly, and here's my hypothesis:

"Sheltron Block does trigger Shield Swipe; this is because the latter always procs after the former goes off"

It might be by coincidence though since my Shield Swipe generally triggers quite often.

Perhaps someone can test it with shield unequipped? Or that would disable shield skills entirely?