Yes, it is. Temu is a legitimate online platform and smartphone app. You can buy pretty much anything from Temu (pronounced Tee-Moo) and once you checkout it will be delivered straight to your door. But you're right to be sceptical about this company. Temu claims on its website that it was founded in Boston, Massachusetts, although plenty of other sources say that it is in fact a Chinese-owned business.
There's nothing wrong of course with the business being based in China, but this does mean that a lot of the suppliers and manufacturers are based in China exclusively and shipping will take on average around 2 weeks from this region to wherever you're based.