The suggestion is totally valid and correct.
First, everyone gets improved by playing more challenging games. Easy games may help a little to maintain confidence (even this might be false confidence), but will not help you improve at all. We are bettering ourselves by going through difficulties. This is like everything else, life, study, or workout. No sweat, no smile.
Second, when you play a game, the end result is averaging both players's strength. The weaker player become stronger, because s/he has to extremely focus and work very hard to survive. Every move must be the best s/he could think of. In the meanwhile the stronger player will become weaker, because his/her imperfect move will not be caught and punished, and s/he still wins when playing badly which makes s/he think playing badly is ok. Next time s/he will make the same imperfect move. Which side do you want to be?
Third, similar to workout, if you always play comfortably, you will get used to the level. It will be very difficult to break through your current level. More workout time will not help. You need some dramatic decision and extra-strong determination to jump out of the box.
Fourth, related the point two, players tend to become careless when they play down, especially when the gap is 200 or 300 more. That's why once in a while we see some big upset.
Fifth, when you play up, if you lose, your rating will go down little, but if you win, your rating will jump. Every win will not only raise your rating, but also boost your confidence. You don't even have to win more games to have a higher rating after a tournament, because your rating change largely depends on the average of your opponents' ratings.
Sixth, when you play up, if you lose badly in a tournament, you won't feel that bad emotionally.
Playing up does not only apply to rating, same with students play in adult tournaments, or girls play in boys' sections, etc. Judit Polgar never plays in Women's tournaments, and Yifan Hou often plays in boys' section (World Youth) or on Men's team. This is how they became No. 1 woman player in the world.