There is actually a developer setting in Instagram to stop swipe up for next reel. You can still watch reels sent by your friends but you won't be able to swipe up. You can also remove reels icon from bottom bar.
If you are using MyIntsa or Revanced Instagram you can enable it (Android only).
Hope it was user option too.
May be they are reading the notification using notifications reading service (Feature in Android) and use intents to send messages.
But from intent you can either share (user action) a message to a particular contact or automatically open whatsapp on a particular contact and prefill the message in text box.
May be whatapp may have exposed an intent to directly send message or they are just prefilling the text box