Yes, he's right.
Imagine this scenario. You happen to be "going to town" - air conditioner is cycled on, dryer running, water heater heating and range/oven has the works turned on. That's 30+30+30+50 = 140 amps* of power from the large appliances.
You are only served by mains power. OK, the 100A main breaker snaps to protect the bus-bars in the panel.
You are served by mains + solar. Your house draws 95 amps from the mains, and 45 amps from the solar+inverter (it's a sunny day). The 100A main breaker does not trip. The 50A solar breaker does not trip. You have 140A moving on that panel with 100A buses.
Now, if the breakers are cleverly arranged (e.g. solar at the bottom, other loads stacked on the left side, sides don't matter) you might still get away with it, with the solar serving the bottom breakers and the mains serving the top breakers - no point along the bus flowing more than 100A. But Code doesn't assume such a happy configuration. Hence the requirement.
Move this over into a subpanel with 200A buses, and now 100A of mains can come through from the mains panel (use a subfeed lug "breaker"), and 50A can come off the solar panels, and still cheerfully power 150A of stuff in this 200A-bussed panel. Happy dappy.
* I'm speaking apocryphally. Most appliances must be provisioned for 125% of their actual draw. Take 24A x 125% = 30A. Appliance builders intentionally size their appliances to just snake under that requirement, so your A/C, water heater, and dryer probably have a 23A load. Realistically 23+23+23+50 (ranges aren't thus derated since they don't expect you to use all spots continuously), so 119 A, etc.