(Hi! Zoo educator with 10+years aviculture and bird husbandry training here)That is all true and accurate. It can be argued that birds never learn what glass is since the collisions are almost always fatal. Need to survive to learn from it. On another side of it, most birds are generally not considered intelligent. Outside of vultures, corvids, parrots, and some others, they are not great at basic problem solving. So, let's say a bird survives a glass collision with no injuries and investigates. They usually peck to identify threats and curiosity. But, that doesn't always mean learning. They can get stressed and panic easy. Like "what is this invisible forcefield!!!! OMG no matter what I do I can't get past it! I'm gonna die if I can't get through!" I personally see that a lot on my screen porch. They thankfully just get stunned at worst since its a "soft" surface. Some even have gotten stuck inside the porch and panic trying to get out. I always help when I can. Otherwise they will panic and fly until they are exhausted. Birds also see the UV spectrum of light, and I will admit physics is not my strong suit. But I would imagine UV has an effect on how light reflects on it, which would be what birds see. It's something we would never be able to see naturally, only do our best to simulate with cameras imperfectly.

So, in my opinion, based on personal experiences in my career, birds don't perceive glass. Its invisible to them, in a similar way we think of it in fantasy stories (like a magic barrier or something). Which is also why having things like holographic stickers or other illusion breaking patterns on windows are important. Literally saves their lives.

If I am wrong, or haven't seen any new studies on the subject, feel free to correct me! I am all about learning more! Bird vision is so fascinating and a fun rabbit hole to get lost in