Copyright © Rogue Amoeba Software, Inc. All rights reserved.
As of spring 2022 and bug fixes included in Apple's HomePod software version 15.4, Airfoil can once again connect to HomePod stereo pairs. Your HomePods should automatically update to the latest software. See this page from Apple for more information.
Just make sure you're using Airfoil 5.11 or higher, and you should be all set. If necessary, visit the Airfoil page to download the latest.