Sigil of Water and Sigil of Air was said to have had their ICD reduced in the feature pack (water went from 10 to 5 sec, Air went from 5 to 3 sec).
I tested Sigil of Water many times in HotM with different multi-hit attacks (within 360 range of the target), but I didn’t once get it to trigger more than once every 10 seconds.
I did also test a combination Sigil of Fire and Sigil of Air on ranger shortbow auto with near 100% crit chance. They did always trigger at about the same time. Although it was harder to measure than Sigil of Water, Sigil of Air did never ever trigger more than once every 5 sec (should be 3 sec).
tl;dr: The internal cooldown of Sigil of Water and Sigil of Air was probably not changed. Please Anet run some tests yourselves and have a look at the code, as RNG-based ICD procs are difficult to test as a player.