I don't think this is Galois theory. Galois theory is about "The Fundamental Theorem of Galois Theory", which states that there is a nice mapping from field-extensions to group-extensions, where the resulting groups are usually finite. When the resulting groups are finite (as they usually are), many problems involving field extensions can be solved using brute-force search.
Galois fields happen to be something else named in honour of Galois.
That quote about equations with rational numbers was from here. Galois didn't have a computer of course. Rational Numbers and trisecting an angle sound related.
The quote is followed immediately by this: "It extends naturally to equations with coefficients *in any field*, but this will not be considered in the simple examples below." Emphasis on 'in any field' is mine. Among the other fields that can be considered include the Galois fields, which are another name for finite fields. (There are also infinite fields other than the rationals, so 'in any field' does not just mean Galois fields/finite fields.) https://en.wikipedia.org/wiki/Finite_field
Galois fields have nothing to do with being able to represent rational numbers in a computer: elements of a finite field aren't even rational numbers.
I can see how the idea extends naturally and also how it doesn't extend naturally. Thanks for explaining, I wonder what Galois would do if alive today with computers.