Super GameBoy = Classic GameBoy (with some extras that have never been seen again, and no link port).
Link's Awakening DX was a Classic GameBoy game (Link's Awakening, of course), with GBC enhancements. Or perhaps, a GBC game that could be played on a regular GB.
The GBC coloring of the game was not available with the Super GameBoy (something which a lot of people could never seem to get a handle on), and the extra color-themed "DX Dungeon" (or whatever it was called) could not be entered with either the Classic GameBoy or the Super GameBoy.
So in a sense, you couldn't really play "Link's Awakening DX" on the Super GameBoy, it would only really be "Link's Awakening".
Except that Link's Awakening DX was also enhanced for the Super GameBoy. It had a special "four shades of green" color palate it would automatically select, and it had special artwork around the border of the screen.
I'd guess that "Fred" was probably just having a bad day, and was tired of trying to explain to people how the older Super GameBoy wasn't designed to play not-yet-invented GameBoy Color products, even though "Yeah but my TV can display a whole bunch of colors. Why can't it play the GBC? The cartrige fits in the slot. It should work but it doesn't. Why can't you fix it? You suck."