" As a general rule (because everything is naturally full of gray areas) it's considered to be impossible for whites to experience institutional racism in countries where we're the majority."

Sorry, what? Considered by whom? Because we should have a word with these people about how unimaginative they are.

For one thing, there are plenty of ways a member of group X could experience institutional racism in an X-majority country. Perhaps they live in a Y-dominated region of the country, or work for a Y-owned company, or group Y has more political/economic/cultural power in society despite being a minority, or they work for an organization whose policy is to hire and promote Y preferentially.

More generally, I don't see the purpose of desperately insisting that institutional racism can never affect group X. To say that group X can be harmed certainly doesn't mean that group Y isn't harmed more. It's a weird and pointless linguistic game that harms the purpose of achieving equality of opportunity by gratuitously driving away allies.