By masking, I mean it will cover up the squeak, but not fix it.
The cause of the squeak is one of four things:
A loose belt (worn)
A warm belt (too hot)
A broken belt tensioner
Any combination of the above.
In order to fix it, the dressing will lubricate and coat the pulley and belt, making the sound go away. That will not fix the looseness or warmness of the belt. It will also not fix a broken tensioner.
Replacing the belt will fix the problem, unless it's the tensioner. In that case, you'd need to get Toyota to fix it.
As for the low mileage, it doesn't really matter whether you have 1,000,000 miles or 1, belts can be faulty, break down, get damaged, etc.
If you got cleaner on it, it might have started to break it down. Salt from the road in the winter does this too, as does extreme levels of hot and cold.
If you are unsure what to do, the dressing is the cheapest and easiest route to go, however it will never fix the problem. My suggestion, would be to see about getting the belt replaced by Toyota, yourself, or someone else who can do it. If Toyota does it, make sure you ask for the felt material belt, if you do it at a shop or with a friend, get a Gatorback belt.
If it's the tensoiner, then that is covered under the 36K/3yr warranty. If you are over that, then you are on your own to get it replaced.