Bird dogs are hard in that you have to keep a neutral spine while moving the extremeties. It's anti extension and anti-rotation and you want it to be symmetrical on both sides. Hips comes into play a lot. Add to that, you can't see if you're swaying, or shifting, or if your pelvis is rotating, so you have to rely on the feel. Best is someone instructing you because what you're feeling is not always what's going on.
This is of course if we really want to be strict about it, but that's the real purpose of the exercise. It's not just lifting your arms and legs. Moving from strength to control is another form of difficulty.
Yeah I think you got the right idea. He's coming from a point of view of lower back pain and is biased in that regard, though most of us will experience periodic episodes of back pain. I also wouldn't say that the cutoff is necessarily 10 seconds or whatever, as long as the quality of the exercise remains. I think that what happen is that people with chronic lower back pain can easily hurt more if you pass a threshold that is often not very apparent. I could also theorise that for practical purposes, most reflexive stabilisation of the core in daily life (to support an activity and reduce pain) happens in shorter durations. I haven't read what he's using to back the 10 seconds up with and I've found myself disagreeing with his study finds before. He did kinda overemphasize the whole "flexion cause disc denegeration" thing a few years ago, which is without context and brought a lot of bad shit with it.
That aside, fun part is when you can hold the plank well and then start moving into different exercises with it. That's the next level of progression.