They're releasing zero D&D editions, since it's all "Just D&D" according to their video.
Because they say it doesn't make it true. They are incentivized to minimize the difference, because, as has been said, they are rightly concerned about splitting the fanbase.
The changes we have seen proposed in the playtest documents so far are greater than the changes from 3.0 and 3.5. And their claims of "backwards compatibility" have not been specific enough to say anything with confidence. We have, however, seen a fundamental re-imagining of how races work, and how feats and ASIs are to be implemented. What, then does this mean?
Here is what I think will be the case:
* adventures from 5e can still be run with 1D&D characters. (= "backwards compatibility")
* 5e characters will be able to be played in new 1D&D adventures (new stat blocks for monsters will still be able to be fought with old characters). Players technically will not need to buy new PHBs (= "backwards compatibility")
* 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. They will not be at the same power level, and 5e players will sometimes wish they had the extra toys that 1D&D will have, but they can coexist (= "backwards compatibility").
* Rule expansions for 5e (esp. Tasha, Xanathar, MotM) will not be 100% compatible with the new PHB. Once they release 1D&D, there will be a market for ever more rules expansions, to fill the gaps for players (esp. for race and subclass options). It will not be possible to mix-and-match player options from 5e and 1D&D, without at-the-table adjustments and house rules.
For each of the first three of these, I think it is possible with a straight face to say that 1D&D is "the same game", and they are backwards compatible. It is the fourth point, however, where I feel they cannot succeed in making it backwards compatible, nor do they want to. They have a fiscal obligation to shareholders to continue to release new books, and the changes we have seen in the playtest appear to be premissed on changes that will leave a window for new subclass, feat, and race options for players. They also have a fiscal obligation to maintain this imprecision, so they can sell books in 2023.
It is also the fourth point that many players, particularly on these boards (based on what people discuss) care most about. If we hold out for backwards compatibility for all player options (something they have not said, again and again), we will be disappointed.
Do I know this? No -- no one does. But I have seen nothing that makes me think otherwise, and the result will still let them say they've fulfilled their promise. I will be happy if I am eventually shown to be mistaken.