I agree with you to certain regarding both the Sendiy and the Magaosi could be the same but let's just consider some other options,
Above is the frequency response chart by the Sendiy whereas below is what i got from the official Magaosi store when i bought my K1 at Taobao.
Just for a comparison though it may not be accurate. If Magaosi being an OEM maker it probably has ODM capabilities whereby they could've tuned according to client needs so probably they're not the same thing afterall.
Plus let's not forget the history on the banned M2 versus the Sendiy M2 where the former sounded nothing like the real deal sound yet they both look 100% the same thing.
I still like my Magaosi K1 anyway, smooth lush midrange which sounds especially great on female vocals, but still it's all bounded by personal preference as i remembered one reputable member didn't find its sound to his liking.
I believe one will only know when one has tried both the Magaosi K1 and the Sendiy M1221.
Just my honest opinion.