Definitely not covered by EC261, the regulation wording is very clear when there isn't an involuntary denied boarding: "except where there are reasonable grounds to deny them boarding, such as reasons of health, safety or security, or inadequate travel documentation".

Having a PCR test which takes more than 48 hours is near useless from an epidemic control perspective, it's an academic exercise at that point. Many countries can do them in 12 to 24 hours, including the UK and Denmark which test at a similar level to the USA.