The announcement isn’t necessarily for a new season, just a sequel, so that leaves open the possibility of a movie. From the article:

Kadokawa announced on Friday that it is producing a sequel anime for KonoSuba – God’s blessing on this wonderful world! 3. The announcement did not reveal the format of the anime. Screenings of the new Kono Subarashii Sekai ni Shukufuku o! 3 -Bonus Stage- original video anime (OVA) also ended with the announcement.

This announcement coincides with the theatrical run of a pair of OVAs:

The Kono Subarashii Sekai ni Shukufuku o! 3 -Bonus Stage- OVA consists of two unaired episodes that started a two-week theatrical screening run in Japan on Friday. The OVA will head to general retail stores on Blu-ray Disc and DVD on April 25.