The reason why Ye Mo didn't make a sneak attack on Ji Zhiyuan was because he knew that with Ji Zhiyuan's golden core state peak stage power, he would at most be able to only injure him. And even that was only a possibility.
But it was different if he attacked the other two. Ji Zhiyuan was attacking towards the spirit sense mark location, so he wouldn't be able to save the two golden core state level seven cultivators in that instant. Secondly, those two were at most on par with Ye Mo in terms of cultivation essence. So Ye Mo should be able to successfully ambush them with a 90% certainty.
Hence Ye Mo immediately acted.
As his sword produced purple swordrays, Ji Zhiyuan was stunned. He didn't think that Ye Mo would be able to find his spirit sense mark and use it to counterattack them.
