Claiming that players from the 1700s would make up for these disadvantages by watching one tournament and glance through some opening theory is ridiculous regardless who claims it.
But Bronstein did not make such a ridiculous claim, and ...
"I think that, after making a hurried study of modern openings, and watching one or two tournaments, the champions of the last century, and indeed the century before that, would very quickly occupy the same place that they occupied when they were alive."
Until a natural comes along with such profound innate ability, he will be forever in a league all by himself.
Nobody is likely to come along and have a notable chess career playing only players of the sort Morphy faced in 1857-8.