How to connect a semi rigid aluminum duct to a rigid aluminum duct
A couple of potential problems:

Sooner or later the duct will have to deal with liquid water running down the interior wall. It'll be condensate from the exhaust gases themselves, or condensate from warm moist outdoor air falling onto a cool pipe, or rain water entering in a storm.. Anyway, the water will pool inside that connection.
Liquid water will turn acidic because of other compounds in the exhaust gases. Are the proposed sealants appropriate for the usage?

Perhaps you could make your own adapter. Crimp some pleats or tucks into the end of the 90 mm duct so that its diameter is reduced. Continue until it shrinks enough to fit inside the 80 mm duct. Slide the 80 mm over the crimped 90 mm, secure with sheet metal screws, and then tape over the joint.
The tucks can be made one at a time with long-nose pliers by grabbing the metal and twisting, but there are pliers made for crimping ducts in this way. (Thanks for the photo, Amazon.)