I recently had my first sleeper journey for a few years, from Edinburgh. I was disappointed, but not surprised, that boarding wasn't allowed until well after the advertised time, even though the stock was at the platform. That is exactly how I remember it from some years ago. The operator has changed in the meantime but the misleading advertising has not. The system made the mistake of emailing me asking for feedback, so they got it; asking them why they tell lies.
Edit: I forgot to mention that the chap on the barrier at Waverley confirmed that delayed boarding is usual.