I'd want to look at the construction of it. As talontsiawd mentioned, they have some type of laminate construction, and it might be 50% or more void (like an interior door).
It's similar to a hallow core door, just with more inside, so it is denser, just not sure what effect that has on strength.
IMO If you feel safe sitting on it, you can put a 10 gallon aquarium on it.
If you feel comfortable sitting on it, 10 gallons is fine. If you have no doubts about standing on it, especially in an awkward situation, I feel 20 is fine (I weight 200lbs). I actually feel that's a good way to judge it, though kind of nonsensical, it has worked for me. Anything larger, then it needs to be made for an aquarium for me.
That said, my 2 actual aquarium stands I had were terribly made. I feel like most furniture has some underestimating going on to account for the person who may not really pay attention. I feel the cheap aquarium stands are built to hold what it can hold, no more.
Rarely do we actually hear about tanks crashing down or furniture failing, kind of like a DIY de-rimming on a rimmed tank. It is only deemed "safe" once enough people try a certain sized tank and then is accepted to be fine. For example, it used to be no more than 5 gallons, then 10. A 20 long, terrible idea. Then people actually tried and many consider that perfectly safe (I have had two 20 longs with the top rim removed). Same with stands, the Ikea Besta is considered a decent stand for a 60CM tank, purely because people tried it out and it work for them. I will say my Ikea Besta feels far stronger than my 20/29 gallon stand did. I would hesitate to stand on my old 29 stand but actually used my Besta as a stool to hang my lights originally.