Do you actually understand the point of Scotland pre-Union being compared to Scotland post-Union?
Darien was, literally, pre-Union Scotland's one and only serious attempt at far-flung colonisation for gain. Hence the purpose of contrasting that with post-Union Imperialism, in which Scotland, was indeed a wholly enthusiastic participant.
I don't know how to spell this out any more simply for you.
Scotland's failed, pre-Union efforts at colonisation, Imperialism, Empire building, however you want to describe it, do not compare in any way to what went on post-Union. It's on a wholly different scale altogether, both in terms of scope, and persistence. This is the entire point. The latter took place under the "banner" of the Union, not under the flag, i.e. banner of Scotland, hence why that's one obvious reason why there is possibly an entirely different perception of the two flags.
That is not "minimising", it's empirical fact. Scots efforts to colonize the Darien Gulf failed miserably and were over in short order. The Empire, which was already underway as an English concern by the time the Darien Scheme took place, persisted for centuries and covered a significant proportion of the globe. Again, historical fact. It's in no way "minimising" anything to highlight the contrast between the two endeavours.