I took a look at a sample document with an image set to Mark as Thumbnail and exported with in5 version 3.8.7 with Open Thumbnails in Lightbox selected. It appears that in Safari on iOS, the lightbox is opening a new window. You should be able to close the new window by clicking the X, but it may be small on the tab. It should be opening in a lightbox like a pop-up with a larger close button. I'm having our developer take a look. I'll let you know what I find out.
i just noticed a typo in the code above here:
despite that the code still worked, i just wanted to let you know