I got the same offer today on my phone as well, but it was priced at +$5 on top of premium. I X'd it out during my commute, but I can't seem to get the offer back.
Here's an earlier thread about the same thing: https://community.spotify.com/t5/Accounts-and-Subscriptions/Hi-fi-lossless-playback-FLAC-offer/m-p/1...
I really wish it was true. Or at least happens soon.