When they are programming and accounting for resources, the more companions they allow, the more variability in the dialogue trees and other dynamic relationship based systems they have in place. This costs money.
Limiting the choices means they can "fix" the resource allocation and make each outcome a uniform algorithm like they did with DOS2.
In programming, less math = cheaper development. It's just numbers.
What he's saying is that since any of the available companions can be chosen before the "cutoff point" in any combination of 3, it makes no difference in resource allocation for dialogue because the dialogue has to account for any potential combination of characters anyway.
