What is the Pixl and MDBT42Q equivalent to puck.js Puck.connect() for BLE

You just use exactly the same code. The puck.js library works fine on any Bluetooth Espruino.

If you tried the first example on https://www.espruino.com/Puck.js+Web+Bluetooth that'd have worked just fine.

The issue is the code that you're running is actually calling the function Puck.light() on the device itself. There is no light sensor on the board you're trying, so it has no Puck.light() function and the call causes an error.

If you replace Puck.light() for something all boards have like E.getTemperature() then it'd be fine.