That would be the absolute most useless way to learn.
Yet 99% of Cantonese speakers learnt to speak it without being taught anything about tones.

What many people from non-tonal languages fail to realize is that tones is not an added accent on a word but, especially in Cantonese, is 50% of the meaning of the word itself. If you pronounce a word or sentence disregarding tone, the english equivalent would be hearing for instance "wa-" without knowing if the speaker means water, walrus, war,... and is absolutely crucial making themselves understood.
...and once that's understood, you can focus on correct pronunciation.

Only by underlining the importance of tone from the very beginning can you quickly overcome that initial obstacle, for many people learning Canto they might find the first 3 months hard getting the tones in there head and recognizing them but after that this hurdle quickly disappears.
I think this holds true for people who are new to Cantonese, less so for those who've been around it for years...

For children however, who learn this by being fully immersed in the language and for whom the brain is much more susceptible to learning new languages they indeed don't learn these tones by having them pointed it out by them but they do learn it subconsciously, that is only possible in full language immersion however and unfortunately for most people not an option.
Rubbish. I know two people who speak Canto fluently and never had any, not even one, formal lessons, let alone studied tones, lol. One is English and the other is Moroccan....both have been here for ten years... I don't know any tones but I can pull off the very basics and I can hear loads.....mimicry is the key...