There's a user who used to work for Harris Tweed on here, I'll tag him if I can find his username again. But I remember him saying in another thread that the Harris tweed labels are not a good way of dating garments--labels were used inconsistently so you might have a box sitting around for years or decades before they start to get used. Even the bolts of tweed, which were serialed, could sit around for a long time before being used. I have a Harris Tweed coat from the 70s/80s but the serial number corresponds to fabric that *may* have manufactured in the 50s lol.
I agree with you on this jacket looking 90s or even more recent--though 80s doesn't seem impossible. Are you able to date the Dunn & Co coat maker tag? That would probably be more reliable