The article you linked refers to events during the war of 1948, when Israel was already formalized. It's establishment up to that point was primarily based on lawful acquisition, not expulsion. When it turned to an all out war, then yes, expulsion took place.

Palestinians still owned most of the land... and buying land doesn't give you the right to rule over the peasants who till it. https://upload.wikimedia.org/wikipedia/commons/thumb/4/46/Pa...

Yes, it was so lawful that the Irgun had to bomb the British and Palestinians to lawfully convince them to hand it to them.