To clarify for the both of you, DirectX is a graphical API, not a graphical engine. OpenGL and Vulkan are the other popular graphical APIs. I'll explain it more simply in a second, but the technical explanation is that they're the methods via which a GPU can be sent commands and data that tell it how to render a scene. A given GPU has to support the API in order for it to work. DX9, DX10, DX11, DX12, OpenGL and Vulkan all work differently, but can mostly be used to achieve the same results, albeit they're usually better at it the newer the API is.
Let's try explaining it... like trying to make a picture on a piece of paper. If you want a replica of the Mona Lisa on a piece of paper, there's so many ways to make that, just like there's so many APIs. You could use a super nice printer, you could use an old printer that's kind of slow, you could have someone screenprint it, or you could even have someone draw it by hand with color pencils and pens. We've had paper and color pencils basically forever, just like DX9, and you could get a very good replica of the Mona Lisa... but it would take FOREVER for someone to do that. If you wanted it done faster, then it'd probably look more like a sketch of the picture, if you had a good enough artist. That artist is like DX9.
DX11 is more like a really nice printer. It can do the same job way faster, but it can also do a much better job faster too. It can print a replica of the Mona Lisa in seconds, and if you had a good artist do it with pencils and compare them side by side, they might look the same. But if you told the artist and the printer to do it in the same amount of time, the artist's picture would look much worse. That's what the difference between DX9 and DX11 is, but obviously less extreme. An API is like the final process that makes the picture.
A rendering engine is different. Think of it like the way you tell someone or something to make the picture. If you want to tell an artist how to draw the Mona Lisa, you could try describing it, or you could show them it on your phone, or a really nice monitor, or show them a different printout of it. If you wanted to tell the printer how to print it, you could use pretty much any computer, any OS, even a phone. You could have it connected over USB, or ethernet, or wireless. You could print from a web browser, or an image saved on your computer, or email or whatever. The engine is what decides how to make the image, the API is what it uses to make it.
That being said, all of this only happens on the computer the game is being played on. If a game engine knows how to render using DX9, OpenGL AND DX11, like this game does, it doesn't matter which it's using, the engine is still using the API to make the same image. No matter which API is in use, the engine still uses the same data to tell the API to make an image. Whether or not you see another truck is not dependent on which API you nor they are using. Their computer is making the image however it can, and yours is making it however it can too. If a game engine made certain things visible under one API and invisible under another, that'd be really bad, and the API where things were invisible would be unusable.
The reason all this matters for TruckersMP is because the mod has to know how to use the same API that the game is using in order to show its own HUD and menus and stuff. It has to draw on the image the same way the game is set to draw its image. If DX11 wasn't supported in MP in the next update, then you wouldn't see any of the MP text or menus when you loaded the game in DX11. That's all. Of course, making MP use a new API is just as hard as it was for SCS to make their game engine use a new API. That's the proper explanation.
Edit: As a side note, the mod currently supports DX9 and OGL. They didn't have to add OpenGL support, but they did. They could have told everyone who wanted to play the mod to use DX9, but they didn't. I'm sure they'll support DX11 as soon as possible, especially since it's that much faster and better than DX9 and GL.