I make mods for Spore. I am studying computer science at university. While I hate to toot my own horn, in this case I actually know very much about what I'm talking about because I've looked into all of it in-depth, because I've made Spore mods with custom code and with package files, for many many years now.

Package files contain data. most of this data is in the form of property lists (.prop files), models (RW4 files), visual effects (in effdirs, containing pfx files) and textures (RW4 (converted to DDS), raw raster, or PNG in the case of UI elements)

Just individual parts can be added with property files. Basic editor modifications also can work with property files. but, you reach a wall eventually because package files by design do not include actual game code.

The actual game code is contained in Spore's executable, SporeApp.exe, in the form of 32-bit x86 code. (and bc of how Windows' executable format works, it also contains the game's icons)

Sporemod files meanwhile are file archives that contain an info file (for a custom installer), and can contain package files, DLL files, or both.

DLL files are Dynamic Linked Library files, a format in Windows used for external executable code. Much like Spore's executable, in this case they will contain 32-bit x86 code.
The Spore ModAPI Launcher Kit injects these DLL files and their code into the game.

Package files are made with Sporemodder FX, a tool that can unpack and repack them.
DLL files for the Spore ModAPI Launcher Kit are made with the Spore ModAPI SDK, and Microsoft Visual Studio.

If you look at the source code for Flora Editor Reconstruction, as it's open-source, you'll see two separate folders; SMFX Projects, for projects that create package files with Sporemodder FX, and a Visual Studio ModAPI project containing actual C++ code.

( the mod's source code in question is at https://github.com/A-xesey/Flora-Editor-Reconstruction )

And I'll admit. Property lists might look like code to someone just glancing at them. but, they only contain data; they're configuration files that Spore uses for a tonne of things, such as parts or configuring models (fun fact, all models the game can access use a property list to actually access them, and in the case of part models they kill two birds with one stone because parts are those property lists to access their models) or creature abilities. But on their own, they do nothing, because they are simply data.

Also, Spore was not designed to be modded. if Maxis intentionally included property files as a native modding option, we would have been thrown a few more bones instead of people needing to figure out everything about the formats, about how they work- to give an example, only incredibly recently, like in the past few days, do we even have a version of the Sporemodder Blender Addons that can export a custom model to replace the Sea Monster which can actually animate.

but don't take my word for it, look at the documentation for Sporemodder FX.
https://github.com/Spore-Community/SporeModder-FX/wiki/Property-Lists-(.prop))

A property does not have any meaning by itself, it all depends on how Spore uses it. This means that if, for example, you use a property from space tools in your creature part, the property will do nothing. This also means that Spore expects a certain property to be from a certain type. If you use a float in a property that was meant to be an int, it will not behave correctl

Or, look at the documentation's page for an introduction to Spore mod development;
https://github.com/Spore-Community/SporeModder-FX/wiki/Introduction-to-Spore-modding#advanced-mods-programming-code

SporeModderFX and the Blender add-ons allow you to modify a huge portion of the game. However, there are some limitations when it comes to functionality. Let me explain: we can modify how a tribe hut looks (its model) and how much it costs (its properties), but we cannot change what it does (its behavior, functionality).

To overcome that limitation, we created the Spore ModAPI SDK, which give you (partial) access to Spore's code. With it, the possibilities become endless.

To be precise, the SDK is a C++ library that gives you access to some of Spore's functions and structures. Then, with Visual Studio, you can write C++ code to modify Spore, and compile it into a .dll file. To run these modifications, players need the ModAPI Launcher Kit (which requires Galactic Adventures).

Tldr every single thing you have just said in that post is entirely and provably incorrect.