And your impression that they are denying any incompatibility is false, because they have said that you may not be able to mix and match on a single character, but you will be able to play 2014 and 2024 characters in the same party. There is an implied “without some mild conversion”, but they’re being cautious to not make the claim you are saying they are making, because people keep hurling insulting verbal abuse at them when they talk about the playtest and the new core books.
We once played OD&D, AD&D, and B/X at the same table for an extended period of time with no trouble. True fact.