I don't think these words even begin to scratch the surface of the experience I'm talking about.

> I don't think similarities between how some people act on those occasions when they are overwhelmed - whether they are overwhelmed rarely or frequently – and how other people act 100% of the time, really have much significance.

It's very significant. Among other things, autism is a lot about what, why and how fast makes you feel overwhelmed (and sensory issues are just tiny little part of it). Learning how to identify when you're starting to get overwhelmed and how to cope with resulting meltdown is important part of therapy. As you noted, the word "autism" describes a pretty broad phenotype, but there are common patterns that simply don't apply to non-autistic majority, and which vary in their intensity across individuals. When everyday interaction makes you overwhelmed already, the difference between "those occasions" and "100% of the time" is not very sharp.