I'm sure some people back then believed in IP as a fundamental right, just as some do today. But the authors of the Constitution manifestly went out of their way to distinguish between physical and intellectual property, and to constrain IP protection in ways that physical property protection is not. I haven't read Locke so I can't speak to that. However:
> we have many of them referring to it as a right
It is a right (re-read the text: "... to secure ... the exclusive Right to...") but it's not a fundamental right. Not all rights are created equal. There are fundamental rights (sometimes called "recognized" rights) which are the inalienable rights to which men are endowed by their Creator (to use the terminology of the Declaration). These exist independent of any law. Then there are other "granted" rights, which exist because a law has been passed. For example, the right to vote is a granted right in the U.S., not a fundamental right. Granted rights can be taken away by changing the law. Recognized rights can't. At least that's the theory.
> we have many of them referring to it as a right
It is a right (re-read the text: "... to secure ... the exclusive Right to...") but it's not a fundamental right. Not all rights are created equal. There are fundamental rights (sometimes called "recognized" rights) which are the inalienable rights to which men are endowed by their Creator (to use the terminology of the Declaration). These exist independent of any law. Then there are other "granted" rights, which exist because a law has been passed. For example, the right to vote is a granted right in the U.S., not a fundamental right. Granted rights can be taken away by changing the law. Recognized rights can't. At least that's the theory.