Another issue 
In the tutorial it's mentioned that the soil moisture sensor should be powered by 5V.
This is incorrect at least for the sensor I received in my kit which is a 3.3v sensor (note the missing SOT23 vreg next to C1 and 0 ohm resistor fitted).
Also on the back of the PCB the VCC pin is labeled as 3.3v.
This actually might be a bit concerning as the RP2040 ADC is powered with 3.3v and just doing some quick testing I measured around 3.8-3.9v on the vout pin with the sensor connected to 5v. Thankfully I disconnected it after I was getting a fixed output of 1023 when reading A0... hopefully nothing was damaged.
The RP2040 datasheet recommended ADC input voltage range is 0 to ADC_AVDD (3.3v).