That's really not true in any meaningful way. Maybe the information is somewhere, but some Stackoverflow answers are so good that they are the best resource on a specific narrow topic that you will find online.
See for example something like [0]. That's the type of answer you would get from a good book or teacher, good luck finding something comparable in the often terse official python docs.
Sure, there are a lot of lazy SO Q/As that sound like someone wasn't doing their homework, but that's far from the only type of content there.
[0] https://stackoverflow.com/questions/100003/what-are-metaclas...
(Edit: another great example is basically anything that concerns advanced usage of matplotlib. That library is so weird and the docs so terrible that working off of SO examples really is the most efficient way to use it.)