I thought that solar panels that old performed much worse or stopped working. Especially considering where the tech was in the 1990s.

"performed much worse" is compared to today's manufactured panels. As an example, a 100w panel in 1992 was likely around 12% efficient. This means "of all the light energy hitting the full panel under perfect light and temperature conditions", 12% of that energy is converted to electricity and would produce 100w. Compare this to a middle-of-the-road panel you'd buy for your house today the efficiency is 21%. Both the old and the new panel's efficiency will go down over the years.

What the article is talking about is how much of the original efficiency is retained over the years in real world tests. Lets say we have a 1992 100w panel from my example above at 12% efficiency. That means under the best possible conditions it would generate 100w. Because of age, the article notes that efficiency has degrade to produce 79.5% of its original rating. Meaning this 1992 100w panel today would generate 79.5w. That's still pretty darn good and useful!