I have seen many occasions where it has been stated that "Robert de Pynkeney signed Magna Carta in 1215".
There are several surviving copies of the Magna Carta and I have personally seen the one which is now kept in Salisbury Cathedral when I went there to find the tombs of Roger Pinckney and his wife Ann.
Magna Carta was NOT signed by Robert, in fact it was not signed by anyone. All that is there is the seal of King John.
More information about the Magna Carta can be found at The British Library web site.
Briefly, King John got very greedy and tried to extract as much as he could out of the Barons. They got extremely annoyed with this and eventually some of them captured London and submitted their demands to King John. John eventually had to do something so the Magna Carta was drafted. The barons and King John famously met at Runnymede on 10th of June 1215.
Magna Carta was granted on 15th June 1215 and the Barons were apeased. It contained written constraints on royal authority in things such as taxation, feudal rights and justice. It also limited unjust or arbitrary behaviour of the King. This was the first time that the power of a King had been limited in any way.
The Barons subsequently renewed their oaths of allegiance to the King.
From what I can gather in various texts, including the amazingly titled The Magna Carta Barons and their American Descendants together with the Pedigrees of the founders of the Order of Runnymede deduced from the Sureties for the Enforcement of the Statutes of the Magna Carta of King John (what a mouthfull !!), it seems that Robert was one of the Barons around at the time and as such would almost certainly have been at Runnymede for the "signing" so the claim is true in spirit if not exactly in fact.