What is the meaning of this? Is it as if to say, that spider is so small or so unspiderlike it's barely a spider? As if you made a cake and it turned out poorly, someone might say "that is hardly a cake!"
I take it to mean 'I think it's really unlikely that that thing is a spider'.
It means the former.
I tried "Det är knappt en spindel" but it was rejected. Of these two terms (knappast knappt), can only knappast be used to indicate probability?
Yes. knappt is used to mean 'slightly less than' or 'barely', 'not quite', 'just under', 'not more than'.
Both work in a sentence like Det tror jag knappast/knappt 'I hardly believe it' though. (much like you can say I can hardly/barely believe it in English)
Why is it "det" and not "den"?
We always use det when we start talking about some new subject that wasn't mentioned before.
Much longer post about this here: https://www.duolingo.com/comment/9708920
It can be both:)
Without context, it needs to be det.
I switched knappast and spindel round and was wrong why is this?