Is this due to not having a load (no pico, just volt meter) attached?
No a multimeter/volt meter has a very high inner resistance (in multiple mega ohm range) as does a Arduino and Pico also have.
Raspberry Pi Pico is a 3,3 volt device. So that means you have to change 5v to 3,3 v. And the voltage divider can be calculated with
https://ohmslawcalculator.com/voltage-divider-calculator
But a voltage divider with the same value of the resistors is has half the voltage in the middle point, but I think that you have already figured that out.
So with a Raspberry Pi Pico should the desired voltage be 3,3/2 = 1.65 volt.
And after reading link it just uses edge detection (RISING) so that should work with the current voltage divider and changing the supply to 3,3 volt.