The biggest difference would be if you looked at early childhood.
NT geeks: same as any NT - engage in social play, imaginative play, no evidence of restrictive and repetitive behaviour, no evidence of impaired social communication, no evidence of impaired imagination, no evidence of sensory sensitivity, no delays in development
ASD: does not do social/imaginative play, constant restrictive and repetitive behaviour - lining, stacking, collecting, and very rigid, has sensory problems, impaired social communication - reserved to the point of no interest in interaction, or talks at others, not to them.
They're nothing alike. The social awkwardness from NT geeks is that their interests are different from their non-nerdy peers, the social awkwardness in ASD is being developmentally behind in social communication. The obsessions may seems similar, but a geek it's a side interest to life which they can stop and start when appropriate, but in ASD it could take up the whole day if a parent allowed it to, and stopping them suddenly won't go well but would be fine in a geek.
The only similarities are social awkwardness and obsessions so at a glance they may look similar, but they're not the same at all.