That does not make sense...
If you can take sections of one and the other, that is compatibility.
I can take weapons vs armor table from 2e and drop it into 4e, 5e, or the playtest... by your account that means 2e is compatable and the same edition.
I can take the background rules from WoD (we did it with vampire but I am sure mage and WW at least work as well) and drop them into 4e, so the d10 White wolf games are compatiable with 4e... and 4e is compatable with 2e and 2e is compatable with 5e 2014 or 2024...
at that point all TTRPG are compatible and there is no need to distinguish between them edition or game
If I use the subclass x of book y and replace subclass w of book z, and it works, it is compatible.
If I use the ranger optional ability and replace the phb ability, it is compatible.
already there is an issue, subclasses (as we have seen so far) already don't port over for every class... the bard still being my go to example... they are close but you will need some house rules to do it, by default the systems are NOT compatible... (Just some issues is the levels you get them at, how they effect a now wildly different set of spells, how they effect a wildly different inspiration mechanic)
If some 3pp or an UA offers a better ranger, and I use that ranger instead of the PHB one, it is compatible.
if you pop out the PHB ranger and drop in "GMforpowergamers new improved ranger for 5e" sure... but if "GMforpowergamers new improved ranger for 5e" comes with new rules for conditions, spells, feats, race... those rules are NOT compatible if the book you buy of mine has all of them tied together as much as you cna pull out a class and drop it in, by default the books are NOT compatible
If I use exhaustion rules from 2024 and replace the 2014 rules with them, it is compatible.
that is the exact opposite of compatoble becuse I can't bring either book and use it, I have to make a note that some or all of my PHB is just not useable
If I use races from 2024 instead of 2014, it is no different than using goliath from mpmotm instead of eepc, so compatible.
If I use a 3pp usb cable instead of the one provoded by huawei, and I can load my smart phone with it (although a bit slower) it is still compatible...
if you break open your PS4 you can add a chip to it that lets you play PS1 games... I would not call a PS4 backward compatible with PS1 (this may be an urban legend but I have heard it from multi sources... then again in the 90s I also heard about the cheat code to take laura crofts cloths off from multi sources too)
I can down load an emulator on my computer and boot up the original legend of zelda from my NES... my computer is not compatible with the NES
being able to house rule and hack something into working just shows it DOESN'T work by default
me and you sit down to play 1 has a 2014 book the other has a 2024book (using what we know now) we need to come to compromise on what rules we are using for what... cause the books themselves do not by default work together.
The only thing that makes remotely sense is, that you can take some rules even from the AD&D rules (like rules for dying) and still see compatibility.
so there is no such thing as edition... and every D&D ever published is compatible... heck I could take the rifts or TORG dying rules too... so every TTRPG ever is compatible...
and when everyone is super no one is... when everything is compatible, the word has no meaning