No, it's not true. It would be done as Fletch says. Seems like it was planned at the synopsis writing time, but hasn't been implemented. But let's see if there's some magic.

A private copy in my $PERL5LIB directory, patched as follows...

--- /usr/lib/perl5/site_perl/5.8.8/MojoX/Session.pm 
+++ /home/shmem/comp/perl/lib/MojoX/Session.pm
@@ -81,7 +81,7 @@
 
 sub flush {
     my $self = shift;
-
+warn "LAST ORDERS. FASTEN SEAT BELTS...\n";
     return unless $self->sid && !$self->_is_flushed;
 
     if ($self->is_expired && $self->_is_stored) {
[download]

... used within this script...

use MojoX::Session;
my $session = MojoX::Session->new;
$session->create();
$session->data('foo' => 'bar'); 
warn $session->data('foo'),$/;
undef $session;
warn "did we land yet?\n";

my $session = MojoX::Session->new;
$session->create();
$session->data('bar' => 'baz'); 
print $session->data('bar'),$/;
$session->flush();
warn "end-of that.\n"
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... produces...

bar
did we land yet?
baz
LAST ORDERS. FASTEN SEAT BELTS...
end-of that.
[download]

... which shows that just undefining $session doesn't call flush(). How could it? there's no DESTROY method.