Power Supply Output Terminals not isolated
I have an 20v power adaptor. But I noticed that the output +ve and -ve
terminals are not isolated.

That's right; they shouldn't be isolated from each other. If they were isolated from each other then forming a circuit by plugging it into a load would cause zero amps to flow. You don't want that from any power supply and you don't want the output terminals to be isolated from each other.
You want the output terminals to be isolated from the input AC terminals but that's a different story.