"Ze" does not mean "I tell ya", it's just an ending particle to indicate a style of speaking. Plenty of characters and people within and without Touhou (including in reality) utilize "ze", as an indicator of a style of speech. It does not mean "I tell ya". Your interpretation is *without* basis, don't mistranslate.
Refer to:
https://www.lingualift.com/blog/japanese-emphatic-particles/
https://en.wiktionary.org/wiki/%E3%81%9C#Particle
https://www.nihongomaster.com/dictionary/entry/96937/ze-zo