Don't be so quick to make such judgments:
There is ample scientific interest in the differences between inertial and gravitational mass [1]. That they seem the same is a profound question known to Einstein. He explicitly assumed they were the same in his equivalence principle [2].
So their equivalence is a postulate, not derived from deeper theories. But one day we may find it is not consistent over time. It most certainly is a deep question in physics.
There are other theories of gravity, taken seriously, as possible future modifications to relativity, such as Brans-Dicke theory [3], that remove this equivalence principle, yet still agree with all observational evidence to date.
Current experimental evidence puts them equivalent to under 1 part in 10^-15 [2], with more experiments still being done.
[1] https://en.wikipedia.org/wiki/Mass#Inertial_vs._gravitationa...
[2] https://en.wikipedia.org/wiki/Equivalence_principle
[3] https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory
In the paragraph after your quote is another point the author almost certainly gets wrong:
>When Einstein realized that a person falling freely would feel weightless, he described the discovery as the happiest thought of his life.
I have read elsewhere that this "happiest thought" was that there is no experiment that someone falling freely could do that would reliably distinguish between being stationary and falling freely, with an emphasis on measurements of the speed of light. I am pretty sure that it was common knowledge among physics PhDs back then that people falling freely feel weightless. It is not like physics PhD are incapable of jumping while noticing the sensations in their bodies when the sensations are a straightforward consequence of Newtonian mechanics.