三人行，必有我师(焉)

I don't think there's a clear-cut default in this case, but I would probably assume the "roger that" meaning unless otherwise indicated by context words (我早就知道了 etc) or said in a sarcastic tone
Good to know!
(Relatedly, 早就 is another of those words that my brain will only translate into Japanese and not English, as in とっくに。)

それなー
(though as a somewhat-related aside, I do find the… Venn-diagram-y overlaps?… between 分かる/知る and 知道/了解 mildly fascinating)
Interested to hear more on both these points/experiences any time. For the latter, my impression is that 了解 is like...to understand the nature of something? while 知道 is like, to have information or acknowledge orders or...? (and then there's 理解 and 明白 and 懂 and 认识, why.)