You are correct with the universal truths in case the following is also true:
- the money which was planned until the planned timeline was used - I would also assume that there is a yes
- after ending the project, Purism would fire or seek a different work for those developers to make more profits - here I would assume that this is not the case
You cannot truly see this project as a fix price project. As several comments state, there is additional money coming in and as I wrote I am not sure, if it was ever intended to drop developers after project finalization. However I am also sure they did calculate some buffer - they knew that there were high risks and the timeline we saw initially probably was the best case - I also was very skeptical about the timeline from the beginning as you can see here: Librem 5 Delivery - Project on track?
So maybe - and most of the posts in this thread are speculation - the earned value of the dollars spent via the developers put on this project, are not as high as if they would work on other stuff, but currently I don’t have the feeling purism is lacking progress nor innovation on other parts of their company - they just deployed librem one, have continously progress with the librem key and from time to time have a new revision or less expansive version of the laptops. So fire developers once the librem 5 is delivered? I don’t think so. It was also planned to maintain the librem 5 after the launch - so I assume that for now everything is fine.
In any case - you should always think that Purism is a SPC and not a profit oriented organisation. For purism maximizing profits is not money but making us as a society profit - at least thats my naive view on this company and that’s also why I trust them and back this project (honestly since Sunday knowing all this despite the fact that I didn’t want to buy a new phone until my current one breaks). That’s why I think that Purism can do decisions you could never do in a profit oriented company - they can do e.g. deficits in the librem 5 or another product and cross-financing it via e.g. the librem one, the laptops or vice versa. More or less like a company owned by the state which is serving the people and not their stakeholders to generate interests. Example: Is it economic to have a school bus which is passing by a town in the hills? No. Does this kid have to pay more? No. Is it the right thing for the government to do so? Yes!
Again off-topic:
I agree that puri.sm is vague on the specifications - hell look at the first lunduke interview with Todd - there we heard about a fullhd display, I think 128gb sd-card and such stuff - they changed to the lower resolution display and less disk space relatively soon not now - I think that this is the best deal puri.sm could make. I am convinced that once the final word about cameras has to be spoken, that they will get a good trade off and have mainly freedom and our interests in mind.
Having a tbd there is mostly there because I assume that they don’t want to make false promises - you as a pmi certified pmp should know what’s this about - expectation management 
I have no intention to start a flame war nor am I affiliated in any way with puri.sm - as I said that’s my naive view on what my impression of purism is after everything I read and viewed on this site and interviews I saw. If I’m wrong in the end that’s ok - but from what they did in the past, I know that they support my idealistic view of how I want the world to be and if my money would not be on that - how could I deserve to live in the world of my dreams?