The choice to define SI such that the constants have known specific values, with the result that the exact meaning of the units is the thing our metrication discovers not the values of those constants - is a really clever trick.
This completes the work of making SI truly universal. Once upon a time all of SI was defined in terms of prototypes, available only locally. In the case of objects like the kilogram and the metre, these were literally prototype physical objects, in some other cases what we measured was the planet we lived on, which is at least available in theory to all its occupants, although not much use if you're in Andromeda. While measuring the second according to its current definition is a bit trickier than estimating its first definition (in terms of a solar day) it's actually possible anywhere in principle.
This completes the work of making SI truly universal. Once upon a time all of SI was defined in terms of prototypes, available only locally. In the case of objects like the kilogram and the metre, these were literally prototype physical objects, in some other cases what we measured was the planet we lived on, which is at least available in theory to all its occupants, although not much use if you're in Andromeda. While measuring the second according to its current definition is a bit trickier than estimating its first definition (in terms of a solar day) it's actually possible anywhere in principle.