O. Sami Saydjari wrote:

The book appears to have been published in 1995. He does not seem to
say anywhere that the advice is unique to radials.

Hydraulic lock on radials without clean kits is well understood, but for
wet sump boxer engines like we generally fly behind the advice not only
makes no sense, it's contrary to Lycoming's current recommendations.
With the oil sump under beneath the engine there's simply no mechanism
for lock to occur, unless you tend to park your aircraft inverted.

About the only mechanism I can imagine for causing something like this
would be on some injected engines, where hitting the boost pump and
ramming the mixture full forward while the engine is stopped will dump
gobs of fuel into the intake runners. While much of it will eventually
come pouring out the air cleaner it's not hard to imagine getting
unlucky and having one cylinder with an intake valve open. Hardly seems
the basis for turning the engine over by hand as a standard procedure.