It's perfectly legitimate to say life stores information using only two "types" of things (AT or GC) which can be oriented one way or another, which is a "binary choice of the type". I explain it that way because it reaches a wider audience (easier to visualize)

However there's always at least one stickler who wants to quibble that the orientation is also a "binary bit" of information. Congratulations, you're him today!!

I did think about that, that’s why I added the second paragraph, namely that what about RNAs? (Since presumably they precede the DNA world) They can self-bind (I don’t know the proper nomenclature in English), but by all means they have 4 bases.

There's two kinds of base pairs, is identical to there's 4 kinds of bases.

RNAs are a single chain. Your point makes no sense.

A code with a 4 letter alphabet would be 2 letter, if it happened to use a parity “bit” or other error correction info?

I never said RNA was built on two types of molecules. I said DNA was.