I too feel a bit dirty saying this, but I actually really like a lot of the w3schools docs - terrible place to learn to code, but great for quickly remembering how do to something that you totally knew at some point but have just forgotten.
I for one, would like to know who does their SEO... they are often at the top of the results for many searches.
By extension - the best SEO guide is almost certainly the first listing for 'SEO guide' on google.
Only if you can assume that they followed their own guide. I could see an SEO company posting a fake guide to make their own techniques more effective relative to any competition using that guide.
I think over time, that wrong information was less detrimental to learning, while still being here and there.
I met too many people over the years who started with w3schools and continued on, so it became clear my sense about it was mostly dogma - if folks turned lemons into lemonade and think fondly of it, who am I to yuck that yum :)