I'm pretty fucking positive I READ IT IN THE INSTRUCTION MANUAL and l-cancelling isnt as much of an advanced technique as you people think it is.
For something to be an advanced technique doesn't mean that it has to be a glitch/bug. It's advanced because it's hard to pull off consistently until you get the hang of it, and even then, each character requires different timing.
So you're wrong, whether they knew about it or not, it's still an advanced technique. Just because it's in the manual doesn't mean it's not advanced. It's an advanced technique, and probably one of the most advanced because it is harder to perform than wavedashing. The only thing that's tougher would be either SHFFLs, Samus's missile cancels, or Fox/Falcos little shine combos.
By the way, I've never owned SSB64 or Melee, so I'm unsure if they mention l-cancelling in either manual or not, but I remember people telling me that it wasn't included in the SSB64 manual. Not saying you're wrong, but just pointing that out.