I appreciate the tag but i'm not into HxH lol, i merely commented on the Meruem fights because in the first one, OPM was involved and recently we had Katakuri. I only know the feats of hisoka and meruem briefly.
I understand your argument, but the second slash was not ISDS:
* The stance was completely different (Zoro's stance and sword positioning js generally consistent).
* It's not even clear it was a Santoryu technique (I heard that in the anime Zoro used two swords).
* It appeared the same as Zoro's next slashes that diced Pica's arm.