"This blog entry analyzes that while Team Fortress 2's sniper and spy classes can be useful in small numbers, a team mostly consisting of these two classes is going to be wiped out pretty easily."
This seems like kind of the opposite of what the trope is supposed to be. The goal of game balance is to make it so that everything is useful in appropriate amounts but not too overpowering. From the description of the trope, it occurs when developers are unable to make something naturally balanced, and artificially limiting the effectiveness of something the more you invest in it.
The situation in Team Fortress is good - you do best when you have a bunch of people working together playing a variety of teams. As a result of a natural balance, teams made entirely of support classes don't do well without someone to help them. If Valve observed that teams made entirely of snipers/spies tended to dominate their opponents, and coded it so that having too many of one class would cause everyone playing that class to have their stats drop, that would be an example of diminishing returns for balance.
Strictly speaking, there is a difference between diminishing returns and diminishing gains.
If each point of a stat gives less benefit but it's just as easy to go from 99-100 as it was to go from 1-2, that's diminishing returns.
If each point of a stat gives the same benefit but it's harder to go from 99-100 than it was to go from 1-2, that's diminishing gains.
The two concepts are often implemented for the same reasons, but there are some significantly different ways they can impact gameplay. This article has a lot of examples of diminishing gains. It may be wise to clarify this distinction, if not give the latter its own page.