superpower of English teachers

Education and the breadth and depth of reading are what matters. To quote a few lines from Emily Dickinson:

I never saw a moor
I never saw the sea
Yet know I how the heather looks,
And what a wave must be.

She knew because she was literate and widely-read. Some native speakers of English are abysmally ignorant, and some are brilliant, while most fall somewhere in between. I haven't the least doubt that the same applies to all speakers of all languages.

Americans are very litigious, a part of the legacy the English thoughtlessly bequeathed to us. Most Americans will use such a disclaimer in all sorts of situations. "Well, i'm no auto mechanic, but maybe you should have your catalytic converter checked." "I'm no doctor, but if you want to lose weight, you have to eat less and you have to get regular exercise." So someone appending a disclaimer to language advice is very likely just practicing the habit of not claiming an expertise which one does not possess.

A native speaker - which language does not matter - usually speaks grammatically correct. This does not mean one has to know why.
In German some rivers are maskuline and some feminin. A German knows how to use it corrext, but as a rule does not know why. As a non native speaker you have to learn it from the grammar.
Most people will know when to use each and every correct, but not everybody can explain why.
If you are not a language teacher or a person who is very interested in grammar/languages you will be able to speak your language grammatically correct, but not be able to explain why.
What is logical in one language might not be logical in another language.
In English you say "What is on TV?" In German one wuuld say "What is in TV"
In Swedish I go in the bookstore but on the library.

It might reflect a bit of uncertainty on the part of the person giving the answer.

They might think they know the answer, but they also might not be completely confident, so they proclaim their lack of expertise so that people will take their answer as if it were a good guess, and not take it as if it were absolutely true.

I'm sure though that getting a university degree in English will give someone greater knowledge of the proper rules than an average person would possess, especially when it comes to the obscure and hard to understand rules.

That's true too. Although as a native English speaker I couldn't use "who" verses "whom" correctly if my life depended on it.

I do know though to always use "who" if I don't know which one I'm supposed to use.

Who is subjective, whom is objective. If you would use he, then use who. If you would use him, then use whom.

Three prepositions are used in Swedish depening on where I shop
I shop at Larson´s - at by family name
I shop on Ikea - on by a brand name
I shop in the bookstore - in when it refers to what kind of store.