Yes, the thinking is that to actually enforce a no-fly zone would require NATO aircraft to attack Russian aircraft that attempt to violate the zone, thus escala

Yes, the thinking is that to actually enforce a no-fly zone would require NATO aircraft to attack Russian aircraft that attempt to violate the zone, thus escalating into a war between NATO and Russia. Supplying weapons or intelligence to Ukraine can be done "under the radar" so to speak, without engaging in actual fighting.