OH WAIT!!! Found this along with the same picture:
"Cement pots: Use water, cement and old towels/rags/tablecloths. Make a "tea" of cement and water, soak cloth til heavy, drape over an upside down container/bucket, arrange as you please. Let cure. If needed coat with more cement to increase strength. Paint/stain as desired. drop potted flowers inside new holder."
So that means the liquid is like tea and it says you can coat it with cement after the form has cured to make it stronger.
Also notice that it says they hold 'POTTED plants, so the plants would not necessarily have contact with the cement form.
This was on Pinterest, hang on a minute and I'll see if I can find the original source and grab us a link to it.
