Will iOS 14.5 mean my HomePod can use Spotify?
There are two separate things to be discussed here:

Streaming from an i-device to a HomePod/Airplay speakers
Using Siri on HomePod to directly play music

The first option does not limit you to use Apple Music. You can actually airplay any sound over to your HomePod. Just select your HomePod in the Airplay settings or hold your phone close to the speaker while its playing music.
However, asking the HomePod via Siri to play music from Spotify is currently not supported and that is apparently due to lack of Spotify supporting it. There has been a feature request at Spotify forums but it didn't reach enough votes.
This also implies that updates on Apple's side won't change anything. 3rd party support was announced on WWDC last year. Even further, iOS updates do not necessarily mean anything about HomePod.
Therefore, I would be skeptical about this feature, although it is possible that support will be added at some point.