Backwards compatibility was always a moving target, and everyone uses a different private understanding of how the term might be implemented.
Here's where I laid out my expectations back in November as to what the term would likely mean. I laid out four bullet points there, and even after the adventures of January, I think these hold:
1. adventures from 5e can still be run with 1D&D characters.
2. 5e characters will be able to be played in new 1D&D adventures.
3. It will be possible for players to have 5e characters alongside 1D&D characters in a party, and for most players, it will run smoothly.
4. Rule expansions for 5e (esp. Tasha, Xanathar, MotM) will not be 100% compatible with the new PHB.
The first three of these fall under what many people would think of as backwards compatible. The fourth is certainly not backwards compatible.
What I didn't account for were two further things, which emerged from the events of January. Neither of these are "backwards compatible" in a normal understanding of the phrase:
5. 1D&D will be operating under a different license than 5e in terms of 3rd party developers
6. there will be "official" rules content available only to subscribers, not in the print PHB (assuming they even make a print PHB)
I would love to be wrong on all of points 4-6.