To be fair, vowel gradation (aka ablaut) is a rather weak relationship – one wouldn’t usually say, eg. that in English i, a, and u are related just because of the conjugation of Germanic strong verbs such as sing/sang/sung.
Ablaut is unrelated in this context though. These vowels arise from a much more recent sound change (conveniently called umlaut, because it gave rise to a bunch of umlauted vowels).
Fair point, it's about back vowels becoming front vowels of the same height.