iPhone X optimized app without iPhone X screenshots

According to the docs, you either need just a 5.5" set, or a 5.5 set _and_ 5.8 for X, so yes, if you don't sumbit 5.8, 5.5 will be shown on X, instead.

Screenshot specifications

You must provide a set of screenshots for all device types. For iPhone, you need one set of screenshots for 5.5-inch display, and for iPad, you need one set for 12.9-inch display.

5.8-inch display assets for iPhone X are optional, and don't scale down to other devices sizes. When you upload iPhone X screenshots to iTunes Connect, they will display with rounded corners.