They were battery kings well before that. CATL is the OG of the LiFePO4 chemistry and developed most of the mass production techniques to make the chemistry feasible. Tesla came along later and just bought lots of their stuff.

I thought tesla used Panasonic exclusively until after they expanded into China with the Shanghai Gigafactory?

Yeah precisely. Tesla only adopted LFP when they started production in China and instead of producing their own batteries like they had to in US they just bought from CATL instead. CATL was already supplying SAIC, JAG, BYD, etc in China well before Tesla landed there.