I think this is mostly poor writing on the part of the author. You being able to "immediately notice" is contingent upon having read the data sheet, which was not linked nor reproduced, but the way it was presented made it seem like the diagram was sufficient. If you were to have read the data sheet, it would have stated up front that the circuit performs four-bit addition. But the page did not facilitate that...
I had to do a lot of navigation to get to the first page of content; which dropped me straight into a 4-bit full adder. To my mind, that's logic circuits, not electronics.
To be fair, most intro-to-electronics pages start with Ohms Law and so on, which to my mind isn't electronics at all - it's basic electricity. This site starts with diodes, which is indeed the entry-point to electronics proper. But jumping in with gates is arse-over-tit; gates are a special case of amplifiers, and I think explaining transistors as amplifiers should precede explaning them as switches.
Regarding navigation: The whole thing is set in something like Courier, and links don't look any different from body text, even when you hover. And I generally expect no more than one click from the homepage to get me to page 1 of the content. A contents page that links to contents-of-the-contents pages seems like a rabbit-warren.
