Honestly feel like that advice can backfire big time. Get too focused on testing every plain rock and you'll miss the actual good stuff sitting right there. Spent a whole day once doing scratch tests on gray junk while my friend just picked up obvious chert and quartz pieces I walked past. Sometimes you just need to look at the whole picture, not get stuck on one trick.
That "simple rock ID skills" point is so true. My buddy almost walked right past what looked like a plain gray river rock at a dig, but a quick scratch test showed it was way too hard. He cracked it open and it was a solid agate nodule with wild purple banding inside. Completely changed how he scans the ground now.