"Zero cost" here means the author is asserting it doesn't add a bunch of extra branches or instructions or whatever (compared to normal addition) by the time it gets compiled down to assembly.

For example there's a slow looking if statement in there:

    if ((_is_neg() && _storage < usmin) ||
        (!_is_neg() && _storage > usmax))

But in theory an optimizing compiler could remove it entirely since it can never be true (on C++20 or above).

Whether the compiler does this in practice depends on a variety of factors, but in general they're fairly good at spotting "obvious" optimizations.