They do release the source mapping of the java version alongside the binary. Which can be used to decompile the whole game back to compilable java file. (And you can fiddle it around starting from there). It's source available if you do own the game. So I say 'basically'.
The bedrock version on the other hand is completely proprietary. With very little extension api exposed. So there is very few mods of them.
That's decompiled source and while, for Java at least, it can be pretty usable, it's actually going to differ in many ways from the original source due to optimization and obfuscation. I'm fairly sure neither Mojang nor Microsoft were particularly pleased about this, indeed their use of obfuscation implies they want to make your life more difficult doing it.
I think your point is that the Java version at least is easy to modify, but that's because it's in Java, not because Microsoft wants you to. "Open source" and even "source available" are terms of art which don't apply here. "Very hackable" seems more appropriate.
Before this exists, there is a community project to de-obfuscate naming back to readable human names. And it usually took about a month after new minecraft version released. But this is no longer needed because mojang now release the actual mapping.
So microsoft(mojang) do want to make modders live easier out of own willing. Not just because it is java. Or they don't really need to publish that at all.
Ok, it seems we're both right in a sense. They do release obfuscation maps to help modders. However, they are adamant that this does not make Minecraft open source. I'd also be a bit surprised if they include in those maps the source of DRM/online activation components (relevant to the original topic).
Also, I looked at one of these things, and it's just class names, method names, and source line/column numbers. There's no comments, no variable or parameter names, no original source lines, and no hints about what optimizations or inline obfuscations were done.
The bedrock version on the other hand is completely proprietary. With very little extension api exposed. So there is very few mods of them.