Talk about semantics escalating :) The word open is like saying professional grade, or robust, or a number of other things that are used because, in context, they fit.
Maya offered more and sooner than everybody else things like listeners, a robust socket insertion point, an entry point into the main loop and control over things like it's main event loop sleep, process priority, exposed the graph to a fairly granular level, and offered early on what 16 years ago was a fairly modern dev model for all kind of nodes in a fashion similar to what the developers themselves would have access to. That gap has closed considerably, and in some cases Maya has been surpassed in "open-ness", or at least in what you can comfortably do (viewport work in Maya is a gigantic pain in the arse in example for certain things, and contexts are weak, but that's architectural, not blackboxing), but all in all, if you decide to be objective rather than argumentative about it, it is the one DCC app out there with the most of its guts exposed to the open air. Softimage had made it to a close second and here and there even surpassed it, and in general while less "open", as things tend to be better organized but abstracted away from the guts, also a helluva lot more pleasurable to work with (at least on windows). Houdini is a well known disgrace with the Boost dependencies and with the HDK being largely uncharted and unexploited, and the wrappers for it being more recent and not much better than a reshuffle. Max is single platform and incomprehensible to humans, and the rest of the apps out there barely cover half the stretch of tasks Maya, Soft and Houdini can tackle. All in all, while at (frequent) times being just as pleasant as dipping your balls in hot tar, Maya is the more "open" of the lot and has been for a while, because the abstractions are very thin, and access has massive surface with a lot of entry points. For what most people think of when the word open it's used in context it can lay an honest claim to being "very open". It can't quite say it's intuitive, or pleasant, or well organized, or even fully featured in some regards, and it's the single most painful software out there to do prototyping work in for a lot of stuff, but it certainly isn't "closed".
