> Congress could reset the term of patents and copyrights at any moment, on any terms. That's a much weaker thing than actual property rights.
Property comes in many forms. There's real estate, and movable property, in the most basic sense of property. But there is also intangible property. You can own a licence for use of part of the radio spectrum in a region. This is a valuable thing and you can sell it or buy it from another licensee. Radio regulation is basically all created by government legislation, and it could probably be changed at any moment, too. There are also financial assets, such as shares or bonds. Corporations themselves are legal fictions, and might be modified by legislation, with associated impacts on the property.
I strictly avoid the term "intellectual property" myself. Because conflating all these senses of property is highly confusing. A car is not a steel mill, which is not a patent, which is not a taxi licence, which is not a share in a corporation, which is not land. We need to carefully distinguish these, in most cases. But in a political science sense, they are property: legally recognized rights, usually transferrable, that claim exclusivity to something, tangible or intangible.