"He said that there is a caveat. He said that a sunni that knows about the imamah concept and still rejects it (which any remotely knowledgeable sunni about shias do btw) will automatically become a kafir in the hereafter because there will be no imam to back him up. So, it seemed to me that he believes that a sunni despite being a "muslim" by name in this world will have his Islam automatically taken away from him in akhira, and he will go to hellfire forever like the kuffar. In a word, "yeah, you are muslim, but will not have salvation". Well, I can't verify if this is what every such shias that say "sunnis are also muslim" believe, but that was rather intriguing. Maybe a shia here will enlighten me. But, again, like I said, there are well-founded, genuine theological reasons for me to think this maybe truly what every such shia believes in their heart. If not exactly it, then something along the lines of"
Lets comment on this shall we.
What do Shias believe about Sunnis and their position in the hereafter. It's very simple. You will have different sayings from different books based on thought and opinion of the Scholar. Here is my opinion and what I think. The Sunnis are Muslims just as we are but there is a difference. Our faith is complete and perfected but theirs isn't. It's down to every individual to seek and search for the truth with all the falsehood that has been spread from one place to the other.
If someone is born a Muslim but in an environment and surrounding where no one prays or fasts because no one has ever bothered to because that's how it's been from the beginning and that's what they're use to and it's become a norm and no one thinks it's an issue. Also they're not aware about the importance of praying and fasting because they haven't been informed is one thing compared to those who are born as Muslims but in an environment and surrounding vice versa. Both are Muslims and their position in Akhira will be different despite being from the same and equal sect and school of thought.
You have Aalim and you have Jahil. Aalim is some educated, informed, knowledgeable, aware and known in a particular matter or field and Jahil is just vice versa. Now Aalim which is a Scholar and Jahil a member of the public or community, if both commit a sin or make an error or mistake will be judged and taken differently.
The same is with belief and faith. Yes our position will be different than them and they believe the same about us because it's about belief and faith. This is nothing secret or hidden and it shouldn't be. It's not about what we think or assume but about belief and faith just like if your friends ask you about their position in the hereafter after according to you and they happen to be non Muslims.