My parents landed on May 29, 2015.
On May 29, 2020 they will be short by 15 days to meet the required 730 days.
Their PR card is valid until Aug 21, 2020.
Would it be Ok for them to meet the 730 days before August 21 and not loose their PR status?
Not sure I fully understand your question. As noted, PR status not strictly linked to card expiry.
The more important question is, are they in Canada now? If so, they should - as much as possible - remain in Canada until certain they are in compliance with the residency obligation, and be cautious about going out of country when uncertain.
The calculation of that may be a bit complicated as the relevant period means older 'in-country' days fall off.
But there are other threads and info on that here, and what steps they may wish to take once fully in compliance about renewing documents etc.
General advice is to renew the PR card only when certain that residency obligation is fully met with some 'buffer.'
Again, expiry of the PR card does not mean that they no longer have permanent resident status.