That the point he is making does not apply to open source, but rather to freeware (which is open source too). Open source can be completely B2B, without releasing it to third parties. Also open source is not free software, so you can also forbid your costumers from sharing the source further.
EDIT: That's crap. I only thought about the aspect of continuing sharing, but if you also don't want your direct customers to know, then of course open source is very much relevant.
The point is "most companies don't publish research and most software is proprietary [because that entails giving up the competitive edge]". I don't see anything in there about price. Also, I think most people would say that if you have to pay an exorbitant license fee to see the source code then the software is still proprietary. I don't see how it's not true that most companies keep their software proprietary to retain a competitive advantage.
It seems you are confusing source-available and open source. One of the criteria of open source is free redistribution. If your license forbids sharing the source then it's not open source.
In practice, if you use an actual open source license in a B2B setting, it's best to assume that your software will become available to the general public (e.g. it just takes one person to publish it on GitHub).
> It seems you are confusing source-available and open source.
Yes, you are right.
> In practice, if you use an actual open source license in a B2B setting, it's best to assume that your software will become available to the general public
If that is your trust level in your customer, you also need to assume, that they would also give away the binary. I don't think you generally need that level of distrust in a customer.