The charitable interpretation is that Epic misunderstood the consequence of being kicked off the App Store and assumed that SIWA would stop working as well. The uncharitable one is that Epic spread FUD so SIWA users would convert to email/password logins and Epic would have a more direct line to them.

Epic's outright lie in their help document makes me lean towards the uncharitable interpretation:

> Apple previously stated they would terminate “Sign In With Apple” support for Epic Games accounts after September 11, 2020, but today provided an indefinite extension.