It is not private, it is merely "reserved". If/when that range opens up for Internet address, you'll be in a world of hurt for having used it.
IPv6 is much more stable on what you can use. fc00::/7 is actually private use.
IPv6 is much more stable on what you can use. fc00::/7 is actually private use.
In short: The market has already decided and it's private. It's far from the first time an unofficial arrangement is the de facto standard.