Wonder if I hade a keyword with 20 difficulty in marketing niche I have made an infographics which I want to rank it on my DA 12 blog.
He probably means keyword difficulty provided by services like Moz, Ahrefs, Serpstat and etc. I'm pretty sure there are plenty of keywords in this niche with difficulty below 20. Question is, is their volume enough and do they fit the subject of your content.
I don't know if it does exist but I don't think that number of competition in a certain keyword is difficult to rank. It's too low, maybe it isn't that hard to rank unless if it had a low average search as well.