The defaults for showIcon and showText are undocumented at https://docs.telerik.com/kendo-ui/api/javascript/ui/breadcrumb/configuration/items.showicon and https://docs.telerik.com/kendo-ui/api/javascript/ui/breadcrumb/configuration/items.showtext respectively.

I understand they are true and false respectively when type is "rootitem" and the opposite otherwise.

More sensible defaults would be along the lines of:

showIcon = !!icon