that's a rather sexist position. someone's wardrobe, if in the entirely reasonable range in which high heels are, should not be dictated by someone else's preferences, particular a boss.
For example, it gets really hot outside where I live in the summer but my office puts the AC at ridiculously low temperatures. For me that means I can't wear shorts in the office because it gets too cold. I wouldn't wear shorts every day and act all surprised about it when nothing has changed nor think anything is sexist or even a personal attack on me.
If they want to wear high heels regardless of this knowledge and have no requirement to wear high heels, I feel no sympathy.
A bigger issue might be that such walking meetings may exclude some disabled employees.