Hi-res can be 512x224, 256x448, or 512x448. The person he's quoting is definitely missing the technical details, but the underlining issue (Canoe doesn't display Hi-res modes correctly, BUT it is emulating them correctly) is true.
Hires mode should be working fine in Retroarch. The problem is there is something hardcoded in the Canoe emulator. As in it's only activating a higher resolution display value for games run from a specific point. It does however emulate the underlying hardware calls properly.
Secret of Mana on the SNESCE is running the game at a higher actual resolution. Any games added to the SnesCE (including copying and pasting SoM from the SNESCE to the space where all games are added) are all being rendered at the default resolution (256x224) then scaled up to 720p.
So what's happening is that Canoe is properly emulating Hi-res mode (In SoM and SD3's case, 512x224), then downscaling it to 256x224 before scaling it back to 720p, causing a loss of detail in the only play hi-res mode really effects most games... the graphic layer with text on it.