I'm not going to say that tennis is the hardest sport ... but please don't confuse learning curve with the overall ease of the sport.
Having played competitively in both, It's MUCH easier to pick up tennis, very true. BUT, to be a truly competitve tennis player (which I hope is what the article was referencing) is definitely not so easy.
Getting a serve consistently in the box is not so hard. Getting it to an exact spot in that box, with the correct spin (or not depending on the opponent), with a pace of over 100mph is not so easy. Returning it is just as much fun. And that's just the first stroke.
IMHO, It's VERY easy to become a tennis player with a rating of 3.5 and below (this is where roughly 80% of your tennis population is). Also most pros aren't so strick in what category you are at this level, they just want you to play.
4.0, which I still think is wickedly simple, is very tough for most people (think consistent serve of 80mph or above, consistent stroke, strong at the net). This is where your tennis pro starts weeding out players. Getting above 5.0 is where it's the most fun.
Those of you who think you can play at that level, please go schedule a match with your local pro to get evaluated.
Physically, while I don't get pounded in tennis like I do hockey (miss that aspect), it's just as demanding on the legs (singles, not doubles). Tennis, a lot like hockey, is about positioning and burst speed. Upper body strength is hockey by far (you need strength for tennis, but it's more about form motion than all out strength).
Mentally, tennis by far. The only position even close in hockey is goalie ... and even they're not involved in every play (like you are in tennis).