Autant vs tant
Pourquoi boit-il autant de bière ? Is this correct? I thought autant was used in comparisons, while tant was used as an intensifier. "As much as..." vs "so much...". Does Pourquoi boit-il tant de bière? say the same thing?
The difference is thin, but it is there. "Autant" can only rightly be used when you compare two quantity together. Sure, in spoken language, people will sometimes use both "autant" and "tant", but it's not because people say it that it is right.
I do not disagree on principle but in the case of this sentence in particular, you can say one or the other. Now for the subtlety: this is simply because there is no comparison here. Only an evaluation of a quantity.