The "clip-on" vernacular regarding contemporary military thermal and NV optics for use inline with standard optics is more relative to the rail mounting system.

A clip-on thermal unit doesn't mount to the optic so much - it mounts more or less coaxially with the optic ahead of it on the rail. Often there's a sleeve/shade/boot between them to minimize light leak etc

I'd expect some sort of similar inline rail or bracket for the Leica thermal sight.

Although there have been direct mounts of night/thermal tech to the objective bell of rifle scopes, the standard for field-durable mounting is via a separate mount or rail.