e.g. in Sums or Products, with

$$ Σ_(i=0)^(20) x² $$

you'd actually pronounce that as "summe für i = 0 bis 20 von x²", which is a good argument for where it might be from.

Yes, though isn't that the same when pronouncing math in English? (So it's a good explanation, but doing the detour via German doesn't give us any extra enlightenment?)

Well I don't know how it's pronounced in English, so I didn't know that, good to know though:)