Sure, the proof in Strichartz is constructive, defining a real number that is the supremum of the set.

LLN (I'm going to assume that the family name is Nguyen) can't do that, because they have no model of the real numbers, so it isn't possible to say that some construct is or isn't a real number.

So this is more a case of "what is a theorem if you have a model is only an axiom if you don't".

If you changed the statement of the completeness axiom from "every nonempty subset A of the real numbers that is bounded above has a unique real least upper bound sup A" to "every nonempty subset A of the rational numbers that is bounded above has a unique real least upper bound r", you'd have the Dedekind cut construction of the real numbers. That's not generally presented as an axiom either.