I don't know what our Prophet saw, so I can't comment about that but if you read Quran, you will see that in many verses, Allah describes the judgment day, heaven and hell as if they already happened. Please check Quran verses 6:27, 6:30, 6:128, 7:44, 18:99, 21:100, 22:22, 23:102-108 and many others.
My understanding is that it already happened from Allah's perspective, but not from our (humans and other creation in this universe) perspective. My understanding is that it all already happened and recorded (e.g. video recordings, server logs, book registries, in a way we can't fully understand) by Allah and that information is available to Allah any time. Please check Quran verses 6:59, 10:61, 36:12, 57,22. Surely Allah can show it to anyone, if Allah wishes. However, we (both living and the dead) are still proceeding in time towards the judgment day and heaven and hell. My understanding is that, from our perspective nobody is in the heaven or hell yet, as the judgment did not took place yet. Please check Quran verses 36:52, 23:112-23:113.
A director of the movie knows what will happen at the end of the movie but a viewer has to go through the movie and reach its end to know it.