The manuf is probably expecting it to be used on a solid fuel appliance and as such, the cleaning requirement would be correct. I would expect otherwise on a gas fired appliance. I'd check with them directly and please post their reply.
Most liners I'm familiar with require an annual inspection for maintaining the warranty and sweeping only as required. Solid fuel requires sweeping at least annually, if not monthly depending upon use/ burning characteristics/ etc. Oil requires sweeping when the burner is not properly setup or operating.
Gas fired liners should never require sweeping but should be inspected annually.