Not quite 'entirely DM dependent' because it still constrained by logical assumptions. Or to put it another way, the legal rules of contractual interpretation are that words should be given the ordinary meaning that would be conveyed to an objective bystander with knowledge of the relevant background.The problem with RAI is its utterly DM dependent. Vague and poorly worded rules left for the dm to figure things out. RAI is a design failure - it leaves players to guess at what they meant. For something like this, its so obviously worded like garbage. Its not hard to see how clumsy it is. DMing 5.5 is gonna be awful because everything becomes more vague and conflicting. Much less using 5.0 material, because its NOT backwards compatible. It requires all sorts of dm decisions about what works, how they interact, which thing this and that. The devs of dnd are just offloading all the work to the dm and cashing their paychecks from writing "new" books that are just different enough to justify calling it 5.5
There was an intention behind the changes. What would help me is someone official explaining what problem it was that the change in wording was intended to fix. Then I can ignore any unintended consequences that go beyondthe ordinary meaning against that background.
Same with the silly interpretation of the two-weapon fighting rules.