"Autant" and "tant" are a bit similar, they are both used for quantity. But :
"Autant" means "As much/many". It's more used for comparisons. Ex : "J'ai autant de pommes que toi." / "I have as many apples as you."
"Tant" means "So much/many". It's more used to show how big the amount is. Ex : "Pourquoi tant de haine ?" / "Why so much hate ?".
It's a misuse in French language, people tend to use "autant" instead of "tant". It's common to hear it, but it's still not proper French. I myself often say "autant" even though I shoud use "tant".
It's because "tant" is seen as more formal than "autant".
I've just made a search in a French ebook I was reading, and I've seen twice "autant" used instead of "tant", on more or less 10 iterations. So I guess even professional writers don't mind about it much either ^^.
You know, it's the same as other languages. The difference between the academic version of the language, and what is used by people is huge.
Anyway, the French you'll speak and write if you immerge yourself in France and what you learn in Duolingo are really not the same.
Doesn't mean it's not helpful. It's just different.
Examples "tant" vs "autant" can be found here: