Frankish vowel shifts
Frankish/Old Dutch th becomes d in (Middle) Dutch, so that can't be right. And Germanic eu becomes ie, not ū or ui, so that can't be right either.
I don't think that's right, either. Germanic *þeutōn would give *thiota in Old Dutch and *diozza in OHG, and the modern words would have been *diet(e) and *Dieß(e). Which is very different from the actually attested words. Dutch tuit can only go back to Germanic *tūt- or *tiuti-/tiutija-.