My issue is that I just don't understand how based on the rules, that
black text is better on the purple than white.
This is to do with the varying types of colour blindness and if people with these can read it.
I have asked several people to get their opinion and everyone agrees,
the white text is better, yet based on color contrast checkers the
better ratio is with black text.
Simply asking people around the office (assuming thats what you did) for their opinion isn't good enough, if it is a requirement to use the accessibility guidelines then you need to ask people that need them. Asking people for their subjective opinion on the colour combination is just asking a fraction of the user base. Likely not the people that actually need these guidelines.
How strict should I follow the WCAG?
This depends on whether you want it to be compliant. As mentioned by Devin, you either follow them or you don't. If you want it to be compliant then you need to follow them, if not, it will not be compliant. There isn't an in-between for this. It either passes or fails compliance with the guidelines.
I know this is to allow better visiblity for those with vision
problems, but if it makes it worse for the majority of users (although
What was this based on? How they thought it looked or readability (both subjective until tested)? or both?
should I still follow the WCAG despite what looks easier to read?
If you want it to be compliant, then yes, you need to follow them. If you don't like how it looks then change it, use different colour combinations and/or text sizes that the brand allows. For example, with larger text that colour combination (white on purple) passes WCAG AA.