I thought that vulnerability was gone since at least September 2019 with the 2nd revision?

It is, but the core vulnerability of the chipset (Tegra X1) still exists AFAIK. Granted, you need to hardmod to jailbreak a Mariko model (launch models can be softmodded), but that's basically the same thing that happened with the 3DS.

It's a different exploit. The original exploit is an issue with the Tegra's recovery mode (RCM), requiring a USB payload to be sent to the Switch at boot every time.

I believe the hardmod is a voltage glitching exploit, as described for the Vita here: https://yifan.lu/images/2019/01/Injecting_Software_Vulnerabi... . It is able to inject code through the onboard memory then load a payload from a memory card rather than USB.