Regardless of 'Strip metadata' checkbox the metadata is not stripped when exporting to Flickr. Instead, the state of 'View photos in browser[...]' determines whether the metadata is stripped or not.

From src/FlickrExport.cs (HandleResponse):
        [...]
   	open = open_check.Active;
	scale = scale_check.Active;
	copy_metadata = open_check.Active;

The last line should probably be:
        copy_metadata = !meta_check.Active;


Furthermore it would seem that stripping metadata has no effect unless you're scaling the image as well. There's some discrepancy between the terminology in the UI (strip metadata) and the code (copy metadata) which probably explains this.