Do you mean intellectually property isn't really property, or do you mean intellectually property doesn't consist of estates in land (“real property”.) The latter is obviously true, but hard to see it's relevance; the former is false. IP is no less property than any other form of intangible personal property.
- Ideas are not scarce goods. They are for all intents and purposes infinite. - Ideas cannot be exclusive. Two people can have the same idea at the same time.
There are only so many apples available, restricted by sunlight and water and nutrients and such. If I hold an apple, another person can't also hold that apple at the same time.
That's why IP isn't property.
IP isn't thoughts and concepts, it's expressions and concrete applications, and “property” is a legal relationship; human laws don't define reality, but they do define legal relationships.