The issue with this mentality of putting off learning for later is that it can be applied to all casual content. So someone forgoes learning their proper abilities and rotation but since no one calls them on their poor performance they complete the msq, complete normal tier raids and the next logical step is Savage.
At this point they have sunk what is likely months (learning the wrong habits btw) into this game and rarely if ever been confronted with any kind of criticism. Since there is no in-game representation of their dps they don't know they are doing anything wrong. They join a Savage party and people rip them apart because they're using Thunder 4 on single targets or whatever... yeah, you better believe I would say something if I was in even a learning party and I watched someone making egregious rotation errors.
Know why? A learning party in enraged content is not there for you to learn your rotation or how your job works . If I get a no-melee RDM or a BRD who doesn't use Straight Shot (both things I have seen in o5s) I am 100% going to tell them they need to work on themselves before entering Savage again.
This is the conundrum. The game does virtually nothing to teach players how to play their class. The game does not allow parsers to show someone how they are messing up. The game will allow you to progress through the entire main story with basically no idea what you are doing as a dps. If we discourage people from giving advice in casual content under the premise of 'it's unnecessary' then allow those same players (who have received no feedback for 70 levels and perhaps days of playtime) to waltz into Savage they are invariably going to have an overwhelmingly negative experience.
Yes, tone matters. You don't have to give your advice in a rude way. That doesn't mean all advice is rude, though. If the person giving the OP advice had opened with 'I know a fair bit about BLM and there are a few things you could do to really increase your dps such as x' it would be better received for sure. That isn't the point though. The point is advice is a net positive and even if the OP had his feathers ruffled by the delivery of it helps him grow into a more competent player I would say it's overall a win.