Searching for 'Tom Morton Shetlandic' on google I was surprised to find this:
To quote briefly from the last one:
Q (Interviewer). Do you think that’s there’s a particular Shetland quality to the writing or is it just that it’s talking about Shetland?
A (Morton). There are Shetlandic qualities to the writing but I think the beauty of it is its insight...
Oh well - transcriptions of interviews are not likely to be reliable. And we must surely assume that other uses of 'Shetlandic' by Morton are sarcasm.