And how could they be good at avoiding air bombardment, but not so good at avoiding artillery bombardment?
Hi! I used to be an artilleryman, so I'll take a crack at this one.
The thing is, artillery has a very quick response time. You spot a target, call it in, and a few minutes later - less than 10 for sure, perhaps less than 2 - shells are landing on it. Most of the casualties in an artillery fire mission (we don't usually talk about bombardments these days) happen in the first few moments, before people have had a chance to get to cover. You also don't get much warning a fire mission is on its way. MAYBE the target will notice the adjusting rounds, MAYBE someone will hear the first rounds whistling through the air before they hit, but even so there just isn't much time. The other thing to consider is that artillery is often used defensively. If the Chinese are going to attack, they will have to leave cover at some point and start to get close to the UN forces. At that point they become vulnerable to artillery, and because of its quick response in any weather conditions the artillery is likely to be employed effectively.
None of this is remotely like a bombing raid by B-29s, or even an air strike by Corsairs or something. Strategic bomber raids are planned days in advance, against targets which are unlikely to move in that time. Air strikes are more responsive, but can still take hours to arrive, by which time the target may have either moved or taken cover. From a low-flying tactical aircraft it can be very difficult to see camouflaged troops, let alone attack them effectively. So it's entirely possible to be good at avoiding "air bombardment" while still being vulnerable to artillery.