That's like saying everything is open source if you can write assembly. Minecraft only ever released binaries and they also went out of their way to use an obfuscator so all of the class and method names are garbage and change every update.
The Mod Coder Pack wasn't developed by Mojang. MCP is the work of the community to reverse engineer and deobfuscate Mojang's binaries. The whole point of creating the MCP in the first place was to get around Mojang's use of tools to break most Java decompilers and try and document and come up with sensible names for all of the classes.
If Mojang had simply compiled and released their game without obfuscating it first then there would be no point to the MCP in the first place.
That doesn't change the fact that Mojang didn't make MCP. If Searge didn't step up and do it there's no reason to believe that Mojang ever would have come up with something similar. Mojang promised for years and years that "the modding API will be released any time now" yet they haven't done anything to materially support the modding community other than to hire Searge and the Bukkit team.
I guess they finally have behavior packs like resource packs for Bedrock edition (used to be Minecraft PE) but that doesn't allow you to write any kind of turing complete code for custom behaviors. That's the closest they've come and all that lets you do is stuff like make pigs behave like a creeper and explode when the player gets too close.
To date it has been 8 YEARS literally the better part of a decade from when Mojang first announced that they were going to release a modding API and they still haven't done anything. Releasing these two libraries as open source is a nice gesture but that doesn't actually do anything for the modding community. The least they could do is just stop obfuscating their releases so that MCP isn't stuck doing a bunch of unnecessary work.
But this isn't the full source code.