Since iOS 10, Apple has provided the support for downloading HLS (m3u8) video for offline viewing.
My question is: Is it necessary that we can only download HLS when
Yes, you can download video stream served over HLS and watch it later.
There is a very straight forward sample app (HLSCatalog) from apple on this. The code is fairly simple. you can find it here - https://developer.apple.com/services-account/download?path=/Developer_Tools/FairPlay_Streaming_Server_SDK_v3.1/FairPlay_Streaming_Server_SDK_v3.1.zip
You can find more about offline HLS streaming here.