I understand your concern. The "Enable Canvas fingerprinting-related quirk" Webkit feature flag is understandably vague and I do not know just which quirk it addresses. However, it and Advanced Fingerprinting Protection do not contradict one another or otherwise cancel their combined effects.
You can read some limited information about AFP here, but thwarting personal information-harvesting and intrusive tracking is a constantly evolving challenge as website developers and SEOs implement increasingly devious ways to circumvent it:
https://webkit.org/blog/15697/private-browsing-2-0/
I surmise the ability to enable or disable the "Canvas fingerprinting-related quirk" is present since its setting (either enabled or disabled) can result in unexpected website behavior. It's not practicable to anticipate exactly how that misbehavior could manifest.