I don't think the different usage is related to an individual group rather context, particularly when referring to numbers.

E.g. 3 is a 1 digit number because that's the number of digits needed to uniquely identify 3. There are infinitely more ways to identify 3 the number with padded 0s but those aren't useful unless you're talking about combinations/sequences of groups of digits (like random numbers or PIN codes) instead of actual number values.

I don't think it's specific to "the prime-enthusiast community". Most people would look at you oddly if you claimed that 3 is a five-digit number.

> 0 is a digit.

It’s not a significant digit[0], though, when it’s leading.

[0] https://en.wikipedia.org/wiki/Significant_figures

I agree -- maybe technically not a digit, but I'd argue it's a UX issue if it is allowed in the leading slot.

But I still love the concept!