Uh, the point was that the definition of "property" not only changes over time, but that it can change quite radically.
The idea seemingly lost on you is that while property in some sense has always been fundamental to civilized orders, it has encompassed very different things in different times and places. Indeed, this remarkable flexibility may be the key to its endurance. And this vital flexibility is EXACTLY what anti-reformists try to obscure, deny, ignore, or otherwise wish away when discussing IP.
In their conception the invention of the Internet is irrelevant. They portray the current scope of property rights as rigid, fixed, ever-lasting, and absolutely supreme. And this particular stalling tactic is nothing new. In fact it's appeared time and time again - really, any time that society has evolved in a way that demands a fundamental reconsideration of what is and is not going to be viewed as private property, with its protection provided by the full force of the state.
Of course, the people who take the absolutist view don't like to discuss this history in detail. And why would they? Saying that the current configuration of rights - which evolved over time - cannot be allowed to evolve any further is a conspicuously indefensible position. So when absolutists raise the issue of "property rights" they are not trying to add anything to the conversation. To the contrary, they are attempting to shut it down. The hope is that residual deference to the once-sensible bounds will kick in reflexively, and people will suddenly stop talking about things that are certain to harm their economic interests and / or social standing if discussed openly.
And that pattern has been a consistent problem every time society has felt the need to redefine property rights. When confronted by the kinds of dishonest and self-serving assholes who would try to shut down or derail conversations of this nature, it's important for people to recognize who and what they're dealing with, and what kinds of tactics will be used against them. It's equally important for them to remember just how threatening and powerful open conversation really is, and why it's protected by the 1st Amendment.
I maintain that the conversation surrounding property rights in America circa 1850 is likely to contain an enormous number of parallels with the conversations surrounding IP today. For people looking for a roadmap on how to handle the recurrent problem of inflexible absolutists, it's a fine source. And it's not the only place where they can find echoes of the same losing arguments about maintaining outdated but profitable arrangements. Indeed, I strongly suspect that close variants of the same arguments will appear in every case where the scope of "property" has become a bone of contention.
Regardless, if you still think there is anything "unserious" or even "dishonest" about this perspective, please elaborate.