Originally, Tesla was offering three battery capacities: 60 kWh, 75 kWh and 90 kWh. But they made a manufacturing decision to only make TWO battery packs, the 75 and 90. People who expressed in the 60 kWh battery pack were well educated that it was really a 75 kWh pack, but limited in software to 60 kWh. Obviously, Tesla tried to up-sell those customers to at least the 75 kWh battery pack, but some people still insisted on the smaller pack.
These are the people who got a bit of a boost. It was only the 60 kWh customers who got it, it's not going to last very long (I hear only through the 16th), and then the 60 kWh cap gets restored. All those customers still have the option of purchasing activation, which will (without any hardware work at all) give them a true 75 kWh battery.
It's also worth noting that Tesla stopped that practice some time ago. None of the cars they sell now are limited in firmware.