つけおいた水ごと洗濯機にとありますが、例えば浴槽にお湯を張ってつけ置きする場合は洗濯物のみでも良いのでしょうか?

It says that you can put all the soaked water in the washing machine, but if you are putting the water in the bathtub with hot water, for example, is it okay to just put the laundry in?

By putting the soaked water into the washing machine, you have the advantage that your hands won't get rough and you don't have to touch the soaked water directly, but of course you can also put just the laundry in it.