There are free public restrooms (at least in my country), which are equivalent of the soup kitchens in your example. What he's proposing though is to NEVER pay for the restroom, even if you can afford it - you should always look for free one instead. He hopes that this form of boycott, if it becomes popular enough, will convince toilet owners to not charge for the use. In the food context, it would mean always eating in soup kitchens in the hopes that food producers will give out food for free (or it will be 100% subsidized by government).

The only difference between universal food and universal toilets seems to be the cost - i.e. it would be much more affordable for government to supply free toilets than free food for everyone. In this view his argument makes sense - both the need to eat and the need to pee when in public are fundamental, so if we can take care of at least one of them (toilets), then it's still better than nothing.