This is one of those cases where a rule-of-thumb has been misinterpreted to be a dichotomy.
The thing about the whole "how many reps for size vs. strength?" debate is that building size and strength are not mutually exclusive. If you build one, you will inevitably build the other.
To your question
No, you are not wasting your time. As long as your are in the gym, lifting weights, and progressing, you are never wasting your time, no matter what your goals are. If you do these things, you will progress in all aspects, including aesthetics.
I can't speak to why coolcicada has opted for these rep ranges over others, but I can attest to the fact that if you want to have noticable progress in size, you have to inevitably build the strength needed to lift weights that will give you size.
In that regard, 5-rep sets have been quite popular because you get to lift heavy weights AND experience a certain amount of hypertrophy while you're at it.
But the biggest factor you need to worry about for the immediate future, is that whichever program you choose, it implements a noticable progression in weight.
Example for beginners
Let's say I'm a beginner, and I want to improve size. And let's say I have the disadvantage of being quite weak. I can bench press the bar for 2-3 reps.
Now, I want to build aesthetics right? So I want big muscles! And I've heard that 8-12 reps is the range I want to be in. So what do I have to do?
Well, I have to get the 15kg bar, and do my 8-12 rep sets. But is that going to give me the aesthetic muscles I want? Of course not! I have no foundation to build aestetics on. Ideally, I should be doing 8-12 reps at MUCH higher weights. But in order to get there, I need to increase my strength by a lot.
So I don't grab the lighter bar. I stick with the heavy bar, and continue doing 3-5 reps for a few months, until I can bench THAT for 8-12 reps. And bam! Now I'm a little bit stronger, and more importantly, a little bit bigger!
Now we rinse and repeat. Now I can do the bar and then some for 2-3 reps. And then I use that to build some strength until I can bench THAT weight for 8-12.
The good news
Ok, so you will be spending some time with lower reps at higher weights, but the good news is that while people say that "this builds strength", they're right AND wrong. It builds both size AND strength.
Doing 8-12 reps is fine and all, but working with higher weights is going to increase the weight at which you can do 8-12 reps. And the more weight you pack on there, the more those 8-12 reps will do for you.
Don't shy away from a program because of the rep-ranges it presents. I don't know what kind of reputation that program has, but if it says it will build aesthetics for you, give it a shot.
3x5 is great, as long as you have a steady increase in weight, and thereby strength. And the size will come with it whether you want it or not.