You're doing it right, but the watch directly booting suggests they still didn't implement this feature in the D2 bootloader or there's a different button (or button combination) to hold. On my F5P when holding the top right button while powering on the watch (or plugging into USB - which powers on the watch), it stays at the blue triangle as long as I keep holding the button. And during that time, I'm seemingly able to flash whatever firmware I want - the watch accepts them all.
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