Unix was developed as a side result of a couple hackers wanting to play a game that wouldn't run well on the systems available, so they ported it to a new system and then wrote an OS for that system as well (because hey, why not). At the time, they were being paid to pretty much screw around with technology by virtue of working at a large research facility funded by a corporation with a government-granted monopoly.
Unix wasn't even officially acknowledged by Bell Labs until a couple years after it was developed, and wasn't marketed for use outside the labs until a few years later.
So, no, I don't think the early development of Unix had much to do with the ability to enforce intellectual property.
The comment was about development, not just "early development"; and it was about development and marketing, not just development.
The value of an operating system derives primarily from the applications that run on it, and this is very much affected by the business case, which includes ongoing development and marketing. Unix had that.
Even for the early development, I wonder if Thompson and Richie (and the rest of the team) would have been employed to create software in general in the first place, if their employer would not have some form of ownership in it... Doesn't sound like how a monopolist thinks to me.
This is not to deride linux (which is great; I'm using it now), but to note the role that copyright plays in securing investment in a platform. Attracting users to a platform requires a lot of investment; it's much, much more than just building the platform technology.
I think a much better example to support your case is Python (also Perl/Ruby/PHP/emacs). Very successful; a platform; and not a replication of a commercial platform. In contrast, many successful open source projects follow commercial projects. Why is this? There's pre-existing demand for it; and having a technical guide and an existing user-base makes it comparatively straightforward and reduces the risk (i.e. compared with starting something entirely new).
I speak as someone who has replicated an existing product; and is now creating something entirely new. I find the latter tremendously more demanding. There's no pre-existng technical guide, and there's pre-existing demand for this particular approach.
Unix wasn't even officially acknowledged by Bell Labs until a couple years after it was developed, and wasn't marketed for use outside the labs until a few years later.
So, no, I don't think the early development of Unix had much to do with the ability to enforce intellectual property.