it was as earnest as statement that all of it "formerly Palestine". it was mandatory area that included also transjordan.

"The whole of Israel was formerly Palestine" was rather ambiguous. If the poster meant "The whole of Israel was formerly Mandatory Palestine", then okay that's accurate, but it seemed like the idea was to draw a connection to what we now call the Palestinian people or the State of Palestine. Mandatory Palestine was a very different entity which governed a more diverse group of people.

first comment on your link: What's important to remember is that at this point (as in prior to 1948) both Jews and Arabs were referred to as Palestinians since it was the common name for the region

besides this i am not sure what exactly this map supposed to show. 30 years before that this area was called "province syria palestina" and was part of ottoman empire for hundreds of years