I disagree because I don’t think Zoro can flow his haki as strongly through his other blades as he can Enma.
I see it like Nanatsu no Tazais sacred treasures. Zoro has a strong haki already; however is limited by how much he can use at once. Enma ignores this limit and allows him to use more ryou than Shusui or Kitetsu or Wado ever allowed him to.
But since one slash is so much stronger than the others it absorbs the other two slashes in his three sword style attacks (like what Zoro said in Thriller Bark) and makes the whole attack a lot stronger.
i havent watched sds so i dont get that part. nevertheless let me to explain what i meant.
i see it as enma dragging out haki forcefully whenever zoro forms that special aura around enma alone. that is why it is being highlighted specifically around enma. it makes his santoryu attacks lopsided around enma. but we've never seen this happen with oden. so zoro is doing something wrong that oden did right.
the training is to refine his haki control and regulation. so my theory is that, oden, by using enma, could effectively force out the same amounts of haki (by increasing his haki on ame no habakiri and reducing his haki on enma - thereby achieving synergy by virtue of his better developed haki control) - launch stronger attacks than his normal level since his average haki output on both of his two swords have increased.
so by the end of zoro's training phase with enma, i think we would stop seeing these spikes of haki on enma alone. zoro's haki control would reach a stage where, on command, he can envelope all of his swords with the same amount of haki (but the amount is significantly higher than what he could muster pre - enma). hence the spikes or special highlight around enma would go away and we'd see the same output out of zoro as a whole.
but yeah this doesnt take into account the "oden's ryou" part and this is still a pocket theory of mine. lets see what direction oda takes this.