Why is Higher Brothers pronouncing 你 as "nie"?
You've got the right direction searching through Chengdu pronunciations.

The pronunciation here is definitely not nie, but just a Sichuanese pronunciation of 你.

《现代汉语方言音库 • 成都话音档》lists 你 as:


  ȵi⁵³


Perhaps is the combination of a /ȵ/ initial with a falling tone on the /i/ that gives it a bit of a "-eh" sound at the end.



The wiktionary page you mention above does hint at this as well:


  (Chengdu, SP): nyi3


The yi combo would give you that sound you're hearing there.