Here is a quote from one site:
Of interest is the 1:1 balun mounted at the input of the tuner (rather
than the output where it would be exposed to highly reactive loads).
The various components are floated above chassis ground. When properly
adjusted, the balun sees a 50 ohm load both at the input from the
transmitter and at the output. Its not a perfect replacement for the
link coupled circuits of yesteryear, but in my experience so far, it
perfoms significantly better than competing tuners employing 4:1
baluns. I've tried a few of these matching networks and not one of them
provided an output to tuned feeders even close to being balanced. The
AT4K-BAL is a leg up on this account.

There are dozens of other sites, but it is all the same info..including
several ARRL articles.
The issue is not lightning protection, as you suggested.
Thanks