It is not true that no plant can absorb phosphorous via the leaves. The plants will take up more foliar applied P if they are P deficient, however. Older leaves may not benefit as much from foliar fertilization either. Not specifically for P, but yes, highly concentrated foliar fertilizer can damage the leaves. Some nutrients aren't well translocated from the leaves to other parts of the plant either. I don't know why one might want to apply fertilizer to flowers?
Don't forget also that plants need more than just NPK. Foliar applications are often used to supply micronutrients, especially when soil application may get "blocked" by the soil pH. It doesn't matter how much NPK you apply if any other nutrient is deficient, any nutrient deficiency will have a negative effect on the plant and limit its performance.
Foliar fertilization can give plants a boost and may get a quicker response than applying to the roots but plants do need a lot of NPK so I believe foliar application is more useful for plants growing in the soil where nutrients are naturally present even if not fertilized. If you were to try and supply all the NPK (and other nutrients) via the leaves to plants in pots then it might be difficult to provide enough without damaging the leaves.
