Yea, it would’ve been nice if they had more of a conversation about it. If Ellie wants to know the truth, sit her down and tell her everything that happened. I don’t agree with all the people saying that Joel considered the idea that FF wouldn’t be able to make a vaccine or about the nuances of how a vaccine could be used to consolidate power. His character just was not shown to contemplate those things. But, there are some factual events and context that occurred while Ellie was asleep that he could’ve told her. And yea, if Joel is supposed to be this more relaxed, more caring, less grumpy, friendly, adjusted person, why wouldn’t he open up about his feelings to Ellie? Would’ve been a great time to show us how much Joel has changed.

Idk, the way they had him say it makes him seem like a sociopath. Like you said, seems like it was written this way for plot purposes.