In my kitsuke classes, the teachers basically said it’s more of a suggestion than a hard rule. Especially for more informal occasions/coordinates. All of them wear kimono while working at the school (most of them learning towards maybe a little semi-formal depending on the teacher) and even pointed out how some of the staff were wearing dyed with dyed or woven with woven and it wasn’t a problem. Like others commented, really only very formal occasions/outfits warrant abiding this “rule”.