I didn't say a rigid system can not score goals. Rather my point was with a more rigid system you'll be better defensively but you'll be less likely to score goals, as you should with a more freedom approach. But with a more freedom approach you'll be more likely to concede. Pro and cons, two sides of a coin. You become harder to beat, but you will score less than you should.
A prime example imo is Spain tiki taka when they won the World Cup. They were very hard to score against, but imo they scored very very little regarding all the attacking talents they got at their disposal. They won the tournament with a serie of 1-0 and they scored very few goals even against some very weak opponent if my memory serves me right.
A rigid system surely can score a lot of goals. It helps your team to be organized with clear patterns of play. But as it's also more predictable, and it takes less risk so in order to score generally you must execute your attacking movements really well, fast and precise so your opponent can't stop it even if they can predict it. In short, imo generally you'd need your players to be vastly superior than the ones you face in order to score a lot of goals with a very rigid system.
Once the gap is not that big imo you can't score that much. That prime Barca was a couple of levels above the rest so we should leave them out here imo. Let's talk about the current City. They're vastly superior against the smaller teams. But against the other top teams you can see they tend to score much less. Imo it's becaus the gap is not that big any more.
About Bruno, I didn't watch him back then at Sporting but a quick google at his time there:
First he was playing mainly as a CAM for Sporting. Second looking at the numbers you'd see his best GA rate was at CAM/SS, by a considerable distance against at CM.
