Yes this is true - all beaches in Mexcio are public - but only up to the high tide mark.
The public can walk along the beach, frolic in the water or otherwise use the beach up to the high tide mark in front of any resort - however if any member of the public tries to park themselves higher on the beach(ie above the high tide mark) in front of any resort then that resorts security team will ask them to move on
There is a misunderstanding of the law. Public beach is not only "up to the high tide mark", but a fringe of 20 meters (65 ft) from the high tide mark. So we can assure that practically all the beaches in front of DRM and DP are PUBLIC. Nobody, included the security guards can tell you a single word if you decide stay all day long at the beach in front of the resort. Of course, we must respect the right of others (including Desire guests) of enjoying the beach without invasión of their privacy.
Here is the transcription of the law which defines the ZOFEMAT, it means: Zona Federal Marítimo-Terrestre
I.- Cuando la costa presente playas, la zona federal marítimo terrestre estará constituida por la
faja de veinte metros de ancho de tierra firme, transitable y contigua a dichas playas o, en su caso, a las riberas de los ríos, desde la desembocadura de éstos en el mar, hasta cien metros río arriba;