This is probably an example of a resumptive pronoun (https://en.wikipedia.org/wiki/Resumptive_pronoun). It could also be an example of anacoluthon, which is the intentional breaking of syntax or grammar for effect (https://en.m.wikipedia.org/wiki/Anacoluthon).
The following is speculation on my part, but it may be useful.
These were used in Old English prose sometimes, and Tolkien was very familiar with Old English. In OE, it seems that the writers might have gotten lost in a clause or two and forgotten what the subject was, so they restate it. We do this in speech a lot.
He may have also used it to add a bit of a stylistic flair to the passage. To my ears, it highlights the speaker returning from the parenthetical and recommitting to the original sentence.