My new Apple TV 4k (2021) remote won't learn my RCA receiver remote

I bought the new ATV4k (2021) and I can't even get the new remote to learn my old RCA receiver (which worked fine on every previous ATV remote). I've paired the old one and the new one to the old ATV and the new ATV. NEW REMOTE WILL NOT TRANSMIT IR to old RCA receiver. It won't even "learn" the signal when I use the RCA receiver volume up & down. I even tried using the older ATV remote (which DOES have the RCA code learned) to "teach" the new ATV (using volume up and down) and tell its new remote. The learning bar filled, but the new remote NEVER transmitted a successful signal to the RCA receiver. I'm returning tmrw to the Apple Store and I'll give another shot - hoping this one was just a lemon, But I've read a bunch of threads on reddit - people saying they spoke to apple and apple said it's a different IR transmitter in the remote.

WHY? But ok. Then WHY doesn't the new apple tv4k (2021) see/learn the IR signal from the RCA remote pointing at it? So it has an older IR receiver too? Something just doesn't add up.

Is there such a thing as OLDER infrared? That doesn't sound right electronically. A signal's a signal, right?