Well I agree to an extent as it's a large and varied game with a lot of secrets and neat little extras, there's more attention to detail and a larger scope than a lot of games even from later eras. But it clearly looks and sounds like the NES based on the aforementioned things, the little stripe with loading tiles near the screen edge when the screen scrolls, the 4-5 channel PSG+lo-fi samples sound, etc.
Top tier NES and 8-bit gen though for sure. Kirby's Adventure also comes close to its level overall, has more animation and background detail, and that rotating tower level which was a cool effect. Off the top of my head I don't know a SMS game with that effect but considering the game Nebulus/Castellan was on NES and various 80s PCs I think it was doable on SMS too.
Limiting it to US SMS releases that does shrink the list a fair bit, yeah, and you're right. It actually had the most releases in a year in 1993 which is kind of crazy! Of course most of those were pretty straightforward "demake" ports from the 16-bit consoles and few games were great.
I'm northern european myself though I had mostly moved on to the MD by 1991 or so and so I discovered some of the games I mentioned later on. While it had good third party support here, we can't understate the effect of the lack of those companies mentioned, along with others like Hudson, Namco for the most part, Sunsoft, SNK, Square and Enix (though the last two were more relevant in JP at the time), and also the licenses these held at the time such as various Disney cartoons, TMNT, and some movies.
The most prolific 80s-90s developers besides Sega and Nintendo were Konami, Capcom, Namco, Taito, Hudson, Compile, Data East, Tose, Falcom, Technos and Tecmo, looking at how many well regarded games they made back then.
Edit: The US did get Sonic 1 SMS which I think beats those two, check out the jungle level for example.
https://www.youtube.com/watch?v=SQPvA0OvR24&t=16m10s