They are referring to the 1948 Palestinian exodus. There are different estimates about the amount of people who left their homes, ranging from 539000 to 935000. Here is the estimate by the UN:
The estimate of the statistical expert, which the Committee believes to be as accurate as circumstances permit, indicates that the refugees from Israel- controlled territory amount to approximately 711,000.
So yes, the number is approximately correct.
The reasons for the exodus are complex and not as clear as presented in the quote. The theory of a planned ethnic cleansing is disputed.
Historian Benny Morris for example says:
Most of Palestine's 700,000 "refugees" fled their homes because of the flail of war (and in the expectation that they would shortly return to their homes on the backs of victorious Arab invaders). But it is also true that there were several dozen sites, including Lydda and Ramla, from which Arab communities were expelled by Jewish troops.
There was no Zionist "plan" or blanket policy of evicting the Arab population, or of "ethnic cleansing".
Other historians such as Ilan Pappé disagree, for example in The 1948 Ethnic Cleansing of Palestine:
Yet when it comes to the dispossession by Israel of the Palestinians in 1948, there is a deep chasm between the reality and the representation. This is most bewildering, and it is difficult to understand how events perpetrated in modern times and witnessed by foreign reporters and UN observers could be systematically denied, not even recognized as historical fact, let alone acknowledged as a crime that needs to be confronted, politically as well as morally. Nonetheless, there is no doubt that the ethnic cleansing of 1948, the most formative event in the modern history of the land of Palestine, has been almost entirely eradicated from the collective global memory and erased from the world’s conscience.
Plan Dalet is often named as showing intent of ethnic cleansing by some historians, while others say that the plan was purely defensive.