This is a definition of the verb "inundate".
to give or send somebody so many things that they cannot deal with them all
I wonder if I can paraphrase it to
to give or send somebody to the extent that they cannot deal with them all
or
to give or send somebody such that they cannot deal with them all
In their usages, I think "so...that" and "such that" both can be used more flexibly in a sentence compared with to the extent that. do they all interchangeable when the sentence behind the phrase comes to describing result or degree?