Part of the problem that isn't addressed in the summary is that to have a cm accurate position you also need to have an oscillator that is accurate in the tens of picoseconds range.
From the article:
> The clock attached to the external front-end was an oven-controlled crystal oscillator (OCXO), which has much greater stability than the low-cost oscillators used to drive GNSS signal sampling within smartphones.
An OCXO is far more expensive than a smartphone manufacturer will happily absorb (~$30). It is also constantly heating the crystal so your battery life gets thrown out the window too.
GPS manufacturers very carefully select their cheaper TCXO chips in order to get nanosecond accuracy. Special tricks are used to get sufficient DAC resolution on the voltage control in order to steer them to the correct level. I have been out of the industry for several years but I would be shocked if there has been a 100x improvement in quality without hearing anything about it.