From the source code of contructor, the "recently" means the connection to server of the instence is recently created, that is the instance was created without a persistent_id parameter or the first to use the persistent_id.
For instance, the gives a bool(true):
$memcached = new Memcached();
$isPristine = $memcached->isPristine();
var_dump($isPristine);
This also gives a bool(true):
$memcached = new Memcached('pid1');
$isPristine = $memcached->isPristine();
var_dump($isPristine);
while this gives a bool(false):
$memcached = new Memcached('pid1');
$memcached2 = new Memcached('pid1');
$isPristine = $memcached2->isPristine();
var_dump($isPristine);