I cannot definitively say that its true, but I'd agree with it. Theres a proven link between stress and a weakened immune system.
For an example, and oscar in a completely bare tank would be bored, as would any animal. Just ask any oscar owner how interactive, inquisitive, curious and intelligent oscars are then think about them with no stimulus. You can even train oscars to do simple tricks eg. swimming through hoops, pulling levers for food etc. Also think about the amount of time/effort zoo's and other animal institutions put into 'environment enrichment' for their animals, hopefully the days of lions or tigers mindlessly pacing up and down in cages is long gone (in this country at least).
I would say to prevent boredom an oscar needs a stimulating environment, by that I mean suitable tank space, some decorations (bogwood or plants say), and tank mates - though choose with care. Sure they might rip up the plants, rearrange the tank but it gives them something to do. I've also seen people float ping pong balls on the surface, make sure they are a suitable size if you try this.