Learn English – “There was no student” or “There were no students”? Which is correct

The difference is in the two different meanings of according to: corresponding with and it is claimed by .

In your first example:

1. If the implementation varies according to platform...

"according to" means "corresponding with", thus:

1. If the implementation varies corresponding with platform...

That is to say, if there are different implementations for different platforms. It does not mean If (a/the) platform claims that implementation varies, which is the other meaning.

Your other example:

2. According to a new design specification from the Military-Industrial Commission in Moscow...

Does not mean "corresponding with", it means "it is claimed by":

2. It is claimed by a new design specification from the Military-Industrial Commission in Moscow...

The difference is that in the "corresponding with" sense, "platform" is not a specific thing, but the property of another thing (a system). So it works like the word "color":

Do you like the color of this dress?

(i.e. The dress has one specific but unspecified color: do you like it?)

I generally like color in clothes.

(i.e. No black-and-white for me, I prefer clothing have color as a property.)

Yes, the sentence is wrong. That website gives some pretty poor guidelines for article usage, but most websites do.

But you did deduce that '*I read book' is wrong.

You usually, but not always, put a determiner before a singular count noun such as book. The indefinite article and definite article are only two of several possible determiners. Other determiners include the personal pronouns and demonstrative pronouns.

'I read books' is rarely going to mean 'all books'. In fact 'books' is most likely going to be a generic noun, saying what type of thing you read. You read 'books' instead of 'magazines' when you have time to read. You can also use 'a book' for the same purpose: 'I read a book now and then to relax'.