> So, it is in fact used that way sometimes then.

No, and that's the point of using a different word "submultiple".

> I've always taken multiplier

"a multiple of" is not the same thing as "multiplier". Or "submultiple" either. Different words have different meanings. So not relevant.

> to possibly mean any positive, negative, or zero value, rational or irrational,

https://www.merriam-webster.com/dictionary/multiple

multiple, noun: the product of a quantity by an integer. So no.