The ProtonMail article said it only applies to pre-existing connections, because iOS doesn't force them to close when an app enables its VPN. I'd be reluctant to call that a leak, unless it contradicts Apple's documented behaviors, which as far as I can tell make no mention of a systemwide VPN except for corporate "always on" ones. Besides, is there any reason why you can't just toggle airplane after enabling a consumer VPN to kill off the old connections? Then it should probably be fine.