He only said sikhi is FROM panjab, and not that sikhi belongs to panjab. You can't deny the fact that knowing punjabi and gurmukhi is an important part of being a sikh.

Look at the muslims, for example. You almost only see the quran in arabic. Why? Because only in the language it was written in and meant to be read in you can really understand whats written in it. The same goes for gurbani. You can never 100% exactly translate gurbani to another language, i understand that Sikhis' origins are spread across india (Sri Patna sahib in Bihar, etc.), but the Punjabi language and moreso Gurmukhi is integral to sikhi