It's not that they made more money from merchandise, it's that they sold more t-shirts than albums. Implying that more people were interested in the "image" of punk rock than the music.
I guess that's the definition of 'iconic' - many a time I have approached someone wearing a Ramones or Motörhead T-shirt trying to chat a bit, only to be told 'Sorry, don't know the music at all, but the shirt is cool...'
Although the article is unsure whether they sold more t-shirts than tickets, implying that people were interested in the music in a live capacity.
Which is a reasonable implication given that punk grew up around the DIY culture. A commercially produced recording doesn't exactly align with the interests of that type of community, even where that community enjoys the music itself.
