It lists harder similar games, and tries not to take too many games with the same gameplay style, which I think is acceptable for an opinion piece like this.

Mushihimesama is harder than Ikaruga. Ninja Gaiden II is harder than Devil May Cry. Battletoads would probably have been a good pick, probably replacing Ghosts and Goblins.

I'd have chosen Boshy, Kamilia or Wings of Vi over IWBTG, but IWBTG is more recognizable.