Hello, feelfreeto!
To answer your question, "low competition" in this guide means low competition in the Search engine results pages (SERPs), not in Adwords or Adsense.
The keyword competition in the Google Keyword Planner (which is for PPC advertisers on Adwords and publishers on Adsense)
is not the same as the keyword competition in the SERPs (this is SEO competition or "ranking competition").
Here is Google's explanation of what the competition level in the Keyword Planner means:
So it's just the number of advertisers who are targeting that keyword in their ads.
As far as if there is any correlation between PPC competition level and SEO competition level, I honestly do not pay attention to the PPC competition level when I'm selecting the keyword as the factors mentioned in part 2 of the guide are what's important (something I omitted from the guide was checking the backlink profile of the sites in the SERPs, although I only do this if glancing over the keyword is not enough), although the keywords I look for tend to be more obscure (under 1,500 searches/month), so most of them have a low Adwords competition level.
With that said, I've seen many keywords with a low Adwords competition levels that are difficult to rank in the SERPs.