It is true that you can read the sensor data from a DHT22 via i2c a gpio pin configured for input using the O2 itself, so it is true that an arduino dock 2 is not required for that purpose.
However the weather station project does more than just read the sensor data from a DHT22 and so is a good introduction to the use of the arduino dock, its library and IDE as well as the IBM Watson IoT platform.
Having said that though, part of the fun of the project book projects is that you can mix and match projects and extend them.
You are of course always encouraged to experiment and to do that safely. ;-)
Welcome and Good luck!!