I think it more comes down to that there isn't really a niche that SuperLemmini fills.
Its primary audience seems to be players who want to play the classic game but with more modern features on a Windows machine. Previously, such players had Windows Lemmings, but this has long been a relatively obsolete incarnation of the game, and was never really ideal given its clunky interface, dodgy mechanics and poorly-implemented MIDI music. That said, the fact it's still talked about shows that it has its place (I'd suggest mostly due to its graphics and the simple fact it's an official Windows product published by DMA/Psygnosis).
SuperLemmini is essentially a vastly improved version of Windows Lemmings: it uses WinLemm graphics, but packages them in a better interface with more features (such as specific level information, the ability to load custom levels, framestepping, replays, and a FF that actually works!)
Mechanics-wise, it seems to be modelled on Amiga, which also makes it a better choice than Windows Lemmings. Why not just play on an Amiga emulator? Because then you don't get the graphic/features upgrades that SL provides.
It also uses Amiga music, which provides players with a more familiar, nostalgic experience (which, again, is likely to be the reason most players discover it initially).
Granted, it's a very small niche, but if someone were to ask me "what's the best way to play Lemmings on a Windows machine?" I'd say SuperLemmini for classic Lemmings, and NeoLemmix for modern Lemmings. And I'd advise them to get both and see them as mutually exclusive products rather than competing products.
In a way, SuperLemmini is a good gateway
to NeoLemmix from (perhaps) DOS/Amiga and (certainly) Windows versions of the game. It introduces framestepping, use of replays and custom content whilst keeping the general gameplay (and, of course, skillset) more familiar. This gives players a way to get accustomed to a more modern way of playing the game before trying out NeoLemmix, which is - let's face it -
much more advanced and more philosophically different in gameplay terms.
As and when SL gets an option for timed bombers, this will become even more true. I'd still recommend all modern players have both SuperLemmini and NeoLemmix - one for classic play, one for modern. There will always be players who prefer one over the other, but it's great that we all have the choice of both worlds!