"The idea that the SSPX would have paraded boys in shorts in its RC Report America in the 1990’s is absurd, since at that time they were also teaching shorts were not appropriate."
So you think the sspx in 1990 more modest than the Catholic Church of the early 1900s?
If you would consider the question you have just asked, you would have perceived the answer was self-evident:
If the SSPX was preaching, until recently, that boys should not wear shorts, whereas you have posted pics of 1900's era Catholic boys in shorts, then it should have been obvious to you that yes, "the sspx in 1990 was more modest than the [American] Catholic Church in the early 1900's.
How so?
Americanism:
The same worldly desire to "fit in" which caused the American clergy not to want to stand out from other Americans, and forego their cassocks, had the same effect on many lay Catholics, who also desired to blend in with their Protestant neighbors.
And in their defense, Testem Benevolentiae didn't condemn this Americanism until 1899.
And when it did so, the Americanist bishops (which was nearly all of them, lef by Archbishop John Ireland in the west, and Gibbons in the east) immediately declared their intention to resist, and in fact they did resist.
Americanism (i.e., the idea of assimilation into mainstream American Protestant culture to prove Catholics could be good Americans) was the prevailing norm, and combined with the lack of any strong declarations from the magisterium until Pius XI in 1928, it is not at all surprising to see dress for young boys lax in comparison to what the SSPX had been preaching.