You're welcome. The code removes Pro's menu custom fields only.
Regarding your other question:
This is a bit of gray area because server limitations and how menu custom fields are handled in WordPress are also involved in this case. When you switch themes, they most probably don't have menu custom fields which our themes use to display a specific icon for the Navigation elements.
Also, only a small percentage of users setup 20 and above menu items. Most sites only have 5 to 10 menus so this was probably not taken into account when the menu custom fields were added. That is just a guess. I am not involved in product development but it would make sense to develop for what's standard or common.
Personally, I have experienced setting up 70 menu items in a popular managed WP host using a popular theme (not our theme) and UberMenu and Appearance > Menus struggles to load. With that said, this issue is not entirely an issue with our theme. This is a general issue with WordPress' menu custom fields and some servers can handle a lot and some don't.
But, there might be a way in the future to officially get around this like deactivating the custom fields when not in use. That's just an example and not to be taken as a guarantee.