many scientists have been puzzled by the existence of stars like the Methuselah that appear to be older than the estimated age of our universe

No, not really, because the age of that star is compatible with the age of the universe within the error bars.

These galaxies, existing a mere 300 million years or so after the Big Bang, appear to have a level of maturity and mass typically associated with billions of years of cosmic evolution.

Galaxy formation is still an active area of research und different models predict different evolutionary rates

Instead, he proposes a constant that accounts for the evolution of the coupling constants.

That’s exactly what Brans-Dicke theory is trying to do, and other modified theories of gravitation as well. Yet they can’t explain stuff so well as Lambda CDM can.

I mean, it’s a bold idea, but I don’t find it so shocking.

It’s well possible that what we call a “fundamental” constant is a variable that depends on other deeper variables. For instance, an earth-bound observer might consider acceleration in freefall to be a constant, but knowledge of universal gravitation tells us it’s a variable that depends on the masses of the objects involved.

It makes sense that other ostensible “fundamental constants” are also dependent on the structure of the universe at any given point in space and time, but the limited window of our observations makes them appear as constants.