Still a few orders-of-magnitude off, though.
Hobbyist electronics manufacturing has always been reasonably affordable, with the old perfboard / stripboard from RadioShack having been a staple of the hobby since forever. At-home etching was also available for the people who cared more about having cool PCBs than about having healthy lungs.
The availability of cheap factory-made low-volume PCBs goes hand-in-hand with the switch towards smaller and smaller SMD parts. You don't need a professional PCB for a few THT parts, and you can always put the odd SOT part on a breakout or dead-bug solder it. But when many interesting parts started to become available only in QFN/DFN (or worse: BGA) packages, proper PCBs became the only option. Combine that with PCB factories wanting to make a few extra bucks off of their wasted board space, and it is suddenly quite attractive to hack together a quick-and-dirty ordering page for hobbyists and prototypes. No new technology needed, just a new market waiting to be served.
$4 / die custom chips are indeed very attractive, but the $4000 MOQ is not. A high-school or college student just getting into electronics can afford spending $100 on a hobby project just to discover it is broken, but there is no way they can afford spending "used car" money on it. That's startup territory, or "retired Bay Area engineer" territory.
Unfortunately they still run into the same basic economic problems: a wafer might be only $1000, but a set of masks is closer to $100k. This means you want to maximize the number of wafers, with ideally each wafer only containing a single copy of each chip. But even if you assume they just fill a reticle to the absolute maximum with unique chips (let's call it 40 unique designs) and fit, say, only 25 copies on a wafer, that's still only 40 wafers, so $2500/wafer in one-off costs - completely dwarfing the actual wafer itself.
For hobbyists you want a process closer to $10k / wafer with zero mask costs. Fit the same 1000 chips on there as before, and you can now sell people 5 copies of their chip for $50. That's a completely different game!