EXCEPT
* his feet are always in goal therefore he is onside and that trumps all
No, not at all.
Rather, his feet are always in-goal and therefore *he's allowed to be off his feet*.
But *he's not allowed to be offside* - regardless of whether he (the player, not the ball) is in-goal.
So (to the extent that anything does) *offside* trumps all - as long as offside exists.
Specifically in this case, he has to remain entirely behind/on the goal line, but he's allowed to have knee/hands on the ground behind/on the goal line - and is allowed to keep them on the ground as he tackles the attacker.
And for what it's worth, once the ruck is over and offside evaporates, he can then place hands/knees on the ground in front of the goal line and still participate in play, as long as part of him is grounded in-goal.
Compare with the equivalent scenario infield, away from the goal line - he must remain behind the offside line of the ruck, but he's not allowed to have knee/hands on the ground and participate in play. Technically, as soon as he lifts said knee/hands to spring forwards he's regained his feet and is back in play - I guess in practice there's some referee judgement involved there. The main thing is that, unlike if he's in-goal, he's not allowed to tackle an advancing ball carrier without first regaining his feet to the ref's satisfaction.
But infield the margins normally aren't as critical, so there's a lot more scope for leeway.
All IMO, as ever.
(And yes, with the advent of the ref mic, commentators' babble is often frustrating - especially when it's in direct contradiction to what the ref can be heard saying.
I watched a number of Six Nations matches with only ref mic and stadium ambience, and hearing refs explain their decisions in real time was a joy.
For the most part I didn't miss the 'special insights' at all.)