How large is the PV system? I can understand the interpretation of why you would need to upsize the service conductors, if we are talking a 200A system. It logically follows that if you need 250 kcmil AL for the PV system, then you would need at least 250 kcmil AL for what it is tapped onto. If it were my choice, I would treat the ampacity of the existing service conductors, outside the context of 310.15(B)(7), as the limit to 1.25*max total inverter current.
By contrast, if the interconnected system uses 175A worth of OCPD or less (i.e. the ampacity of the #4/0AL conductors), I see no reason to increase the service conductors. Simply follow Kirchhoff's current law, and determine whether you add a possibility for overload in the blindspot of the OCPD's. The combination of source and load currents will be subtractive utility-side of the tap point. And for customer-side of the tap point, only the mix of loads on the service panel governs the load on the section of conductor. It complies prior to the PV, and it would comply after it as well.