The “its an issue when it’s separate” line was about RPOs. Not about cars. So if Doria is standing away from Meghan at an event and has an RPO with her, that’s an issue. But if the RPO is with Meghan and Doria happens to be standing next to her, that’s fine.
The transportation thing, is another issue. Harry and Meghan’s travel is taxpayer funded when they’re at work because they are public representatives. Doria is not entitled to use those means of transportation. Harry and Meghan’s private vehicles that they use in their private time are not taxpayer funded and Doria could totally ride in one of those. So her riding in the car with Harry and Meghan is her using taxpayer funded means of travel which is not okay. What should have happened is that she should have arrived separately from them in one of Harry’s private cars. Does that make sense?