It is true. I never thought I'd see that on the Wii and even less from Nintendo itself.
Source?
This is interesting since the hardware is incapable of reading dual layer recordables out of the box, though it's most likely just that it's tougher to read a dual layer burned disc than a pressed one it could also be a firmware update upping the laser intensity.
Actually, found a source confirming smash to be a dual layer disc, and it may well be a firmware update that allows it to be read. Apparently some people are having trouble even running retail copies of brawl on their modded systems, and since mod chips subvert the normal operation of the drive, I suppose functionality could have been added (dual layer support) that not all mod chips are capable of supporting.
It'd be interesting if Nintendo could play around with the drive firmware in order to make mod chips incompatible. Would it be unrealistic to change the firmware to read discs outside of standard DVD specs? Maybe give the laser the ability to narrow its focus to read denser partitions on a disc, or use non-standard sized discs? Triple layer dvds?