Where did Rav Moshe ever mention avsha milsa? He mentioned zilusa d’Shabbos, not avsha milsa.
And yes, the normal cycling on and off, for example, of a compressor, is a keili left on, but a timer is not. Rav Moshe clearly addresses that a timer is not an appliance left on – “שאין להחשיבו למעשיו עכשיו בערב שבת”.