Flags are a bit of a leap of imagination, without the least evidence I agree. But if they had huge great timber posts then that they carved them and coloured them seems quite a reasonable guess. It fits rather better with the finding of ritual deposits in the post holes hundreds of years later, to commemorate the post had once been there. If each post had been a spectacular work of art, or even a deity, that would explain the longevity of reverence towards it more convincingly.