Exactly – if our constraints are reification of our assumptions about reality, making zero assumptions about reality should remove all constraints. Yet, if nothing else, death remains even if you assume nothing. There are many reasons why people would like to be immortal on the phenomenal level, that was not my point however. I used the example as a demonstration that there are well known constraints that human cannot overcome no matter their beliefs, assumptions, or lack of thereof.
Of course, everyone is free to doubt it and try to demonstrate that human can exist unconstrained. That's the usual notion of freedom after all, and there is nothing exceptional about it – humanity is basically doing it as we speak (and I pointed that fact in the article, too). But as I also said, not accepting the constraints that remain no matter whether you believe in them or not is the way of Sisyphus and the source of suffering (and the very reason why human societies are in the state they are in).
Furthermore, while you treat death with indifference, it amounts to the same as considering yourself immortal on the phenomenal level, because with such attitude death practically does not matter. Accepting mortality means that you let it fully influence you so you are palpably aware of the graveness and urgency of your predicament.
If you are honest with yourself, you cannot be sure what is beyond the threshold (or if there even is anything), and you certainly don't know for sure how much time you have left. One must accept even the worst possibility first – not hoping for the best or even acceptable outcome – and only then “not treat that as a tragedy.”
So yes, one should be unperturbed, but death must be ever-present, not farther than a hairbreadth so to speak. Otherwise it's all just empty talk. But I agree that being indifferent toward death creates a lot of space to engage even in fruitless speculations (and corresponding behavior) with no care in the world. That however is no freedom in my book. I see it more as arrogance or recklessness, and, frankly, I have neither time nor luxury for that. I mean, even though no one can be prepared for death – it's a once-in-a-lifetime thing after all – I'd still rather be safe than sorry ;)
