What do you mean it's not. What else would you categorize it as?
It's similar to how "Yookay" is now a word that has certain overtones that aren't conveyed by writing "UK" which is fewer letters.
Britain is ambiguous. Great Britain is geographically specific. Britland is shorter.
Britain is specific, as is Great Britain (you can check.. they're synonyms). Britland is a longer word than Britain, and not used.. unless you count the Urban Dictionary etc.
The Hebrides are arguably British. They are not part of Great Britain.
The sources are about the United Kingdom[0], which isn't Great Britain either.. the slang Britland doesn't distinguish between any of the above, other than sounding derogatory.