Are you asking if the most recommended CE device is legit? 
Or just specifically the listings on Aliexpress? Those prices seems pretty typical from when I last checked.
But yes, the AM6b Plus is one of the “best” UHD drive replacement boxes. It’s not the best “streaming” box, because it’s unlicensed and therefore won’t have access to apps such as Netflix, etc. But if you want to use it solely for CE to watch your UHD collection with Dolby Vision (as long as you install the dovi.ko file correctly), then yes it’s highly recommended.
You can check this thread for more details: Best device for Coreelec in 2024