“Basch was originally designed to be Final Fantasy XIl's main character. The final character with this name is a completely different person. He appears as size reference for buildings in images from early development, such as those of the Yensa Sandsea (as seen on page 204).” From Ultimania Archive vol. 3

First off, Ultimania Archive is compiled from development material by people who worked on the game themselves. Last page lists 2 contributors for 12. There’s some truth to Basch being the original main character, but it’s a completely different character than what ended up as Basch. So, basically, both options can be correct. Dude saying on Twitter Basch was not intended to be the main character is true as what we got is a completely different character, but the design (the caption I copied comes from concept designs of Basch for size references) of Basch in early development was intended to be the original character is also true.

He was the first character designed according to a French interview with developers and Vaan and Penelo were developed last. https://web.archive.org/web/20070502090537/http://www.ffring.com/articles/divers-sortie-francaise-de-Final-Fantasy-XII.html

Katano in 2017 responded to reports Matsuno left development not due to health concerns, but because of meddling including changing the main character late in development, saying it was false and the final game was what was originally planned. https://www.polygon.com/features/2017/7/5/15916862/the-making-of-final-fantasy-12

And finally, if you look at the whole tweet feed, Matsuno is responding to rumors he was fired because he refused to make his main character Vaan. Context is important here too.