After working in automotive, this is less impressive than it appears.

Tons of dimensions on 100k/yr injection molded(and otherwise) parts have similar dimensions. (Although admittedly, after testing in pre-production, I don't know if they are tested again and have drift)

Lego has been making the same parts for decades and their parts are extremely simple. I imagine their 1-off parts for intellectual property based sets do not have this requirement.

I think Lego has a huge incentive to promote this idea that they are high quality to justify the enormous price of decades old technology.

The reason this is impressive has less to do with the tolerances themselves and more to do with backward compatibility across decades at scale. That's the genuinely hard part.

The history here is deeper than most people realize. The United States spent fifty years (roughly 1800 to 1853) at the Springfield and Harper's Ferry armories trying to achieve what LEGO now does routinely: parts manufactured to tight enough tolerances that they are truly interchangeable without fitting. In 1853, a visiting British inspector randomly selected ten muskets made in ten different years, disassembled them, mixed the parts, and reassembled ten functional muskets using only a screwdriver. Tolerances of a thousandth of an inch. It was considered impossible by most of the engineering establishment of the time.

The way they got there was by building machines, then using the parts those machines made to build better machines, then using those improved parts to build even better machines. A virtuous circle of transferring skill from human hands to tooling. This is the actual origin story of what historians call the American System of Manufacture, and it's the foundation the entire modern automotive supply chain sits on.

So yes, any competent injection molder holds tight tolerances today. But that's precisely the point: the reason it seems unremarkable now is that two centuries of compounding precision made it so

After working both in automotive and at LEGO, I think LEGO is more impressive tolerance wise when it comes to molds, molding and tolerance quality control.

Also to correct you, LEGO has been making most of the parts for decades, some have had changes due to new materials (which you can read upon online) but besides the ones that remained the same (not really), many new system elements got released in the last decades and new I.P tied elements get released on a yearly basis.

Ill admit that their parts do have higher quality than their competitors (various Chinese and other companies) making similar or compatible parts - some have injection molding blemishes or whatever on them that I've purchased from AliExpress or Walmart so in this space they are above everyone else in their space.

Agreed. Same or greater injection molding challenges for bottle caps, small plastic containers, things that also are in the hundreds of millions of parts annually. More challenging as they are often using polypropylene which is harder to mold due to its high anisotropy (shrinks in different rates depending on if it's flow or cross-flow direction).