Not to mention that this is a very recent definition of what it is for something to exist, formulated by logical positivists of the 19th and 20th centuries, it also would nullify many of the concepts that Godlessons wants to use to disprove the existence of God, namely the laws of logic. Do the laws of logic exist? If they do, then they are not material objects, rather they are ideas. They are what philosophers refer to as abstract objects. They obviously exist, because they determine how we determine truth. But they aren't physical objects.
Mind-body dualists also have no issue with thinking that immaterial beings exist, as we are partly such beings. Not to mention that objects we conceive of in our minds, such as pink elephants, don't exist materially, but within the immaterial area of our minds.
Also, the proposition, "everything we know to exist has physical properties," itself is an immaterial object. Propositions exist whether we bring them materially into being by typing or writing them down. The proposition 2+2=4 would exist whether or not anyone else did or not.
Another bad atheist argument.