Regarding what you said about Shanks' haki in Wano being a concentrated attack requiring great focus, I can see it happening if he evolved such a technique specifically to counter Teach.
But if he could really pull BS like that Oda would've made GB confirm that his DF was being turned off via dialogue IMO.
Personally I still think he just used a massive CoC burst to warn Ryokugyu that he and his crew were there.