But still.... a 120 inch span (10') of a single floor joist would be
about 1/3rd inch up-and-down play... that seems (to me) like a heck of a
lot of movement... tile fooling or not!

Although what they're talking about is deflection, not bounce, it's
still a lot of movement. But that's why many builders work to
L/480 or higher. Like most of the rest of the code, L/360 is a
MINIMUM standard. If you build to just barely meet code is all
respects, you should end up with a crappy house that's livable,
and not excessively dangerous to you or your community.