With Canon cameras ETTL 2 flash is not focus point based. ETTL was focus point based but Canon changed that in version 2 so you could focus and re-compose and flash exposure would not be effected. In ETTL 2 Flash exposure is based on the reflected light from the pre-flash. I used to have an excellent Canon Master Craft video about ETTL 2 that explained it but like many other articles, blogs, etc that link is also broken by Canon.
You can lock flash exposure but it only meters within the spot metering circle. I found it extremely annoying at events because the flash would fire and I had to tell people that wasn't the actual shot so I just stopped using it.