This person is supposed to be a professional writer?

In an SLM machine you have a cavity that is filled with a fine metal powder, the laser selectively melts/sinters parts of that, the base of the cavity moves down slightly and more powder is added to the top. Rinse and repeat until you have a solid part in the cavity surrounded by the powder that wasn't touched by the laser. I'm not going to cop-out and give a link to wikipedia, but if you go there and only look at the block diagram you'll see why the bed needs to be moved with precision at least equal to the minimum feature size that you're aiming for.

What this project aims to achieve is akin to 3D printed plastics where the raw material is deposited right where it needs to be instead of selectively converting parts of a much larger amount of feedstock. I suppose you could move the laser if you want, but it on the face of it I think it would be simpler to just use a few mirrors attached to servos to direct the beam where it needs to go.

Not to me, and I am one of them.
"One of them", however, gives me pause as the us vs. them mentality always seems to end badly.

The top of his blog post says "using ... a TIG machine". TIG welding is not the same as MIG welding: with MIG the wire is one of the electrodes and is fed using a variable speed motor through the torch, with TIG the wire is held separately in one hand in much the same way as it is with brazing and the torch is in the other hand. I think he's used parts of a TIG welding machine because that's what he happened to have. There's no reason I can think of why the same couldn't be accomplished using a MIG welder.

Of course this would be easily settled if the video showed the machine in action or the blog had any sort of description or diagram of how the machine works on either of the two pages.

Welding identical parts with predictable properties in the same places is one thing, and robots are ideal for it*. So long as there's a need for custom fabrication and patching damaged parts there will be work for human welders.

*Probably why many earlier robots were used for just that, although spot-welding rather than arc.

I'm not sure what you're saying here. If you mean that the word "sintering" is used mainly when plastics are involved then I'm afraid you're mistaken. It's used when referring to sintering as opposed to melting; melting and sintering are quite different and produce very different finished parts. The question of whether plastics or metals are normally sintered or melted is unrelated.

Because the bed holding the part and the powder that's going to be sintered needs to be moved with very good precision. It also needs to be at least as deep as the part you're making. It's generally very difficult to get large things to move small distances.