Probably.

It's 'cause the sub only lists the base game, and thus the API only sends out data on the base game, which SG determines we own, and thus it assumes we own the entire package. If they're not sending out the data on the included DLCs, SG can't determine they exist as part of the package.
So, while in this case the DLC content simply doesn't seem to exist* seperately on Steam (and thus can't be listed to begin with), it can also happen with other games that do have their DLCs available. Life is Strange - Full Season used to have the same issue, for example, until they updated it.

*Exist as listed content, that is. Further, there's an upgrade pack, which contains the same seperate DLCs that the Galaxy Edition has, but the Galaxy Edition doesn't directly use the Upgrade Pack; Meaning that it could be listed, but the need for it is currently bypassed in the sub by the GE directly having the content itself.

The Stellaris Galaxy Edition includes:

Original Soundtrack with 140 minutes of music
The Stellaris soundtrack delivers two and a half hours of original music, including bonus tracks and alternate versions not included in the game. Composed by Andreas Waldetoft with appearances by the Brandenburg State Orchestra, listeners will hear themes meant to evoke discovery and far-reaching exploration through the vast expanse of space through the fusion of orchestral and electronic music. MP3 and lossless FLAC are included.

Exclusive alien race (cosmetic DLC)

Exclusive 130-page Digital Collectorís Book
Join the creative team behind Stellaris to learn how the game's aesthetic design was designed and realized for Paradox's most visually unique game to date. From concept art all the way through full illustrations and 3D renderings, this book includes a collection of game art unavailable anywhere else -- along with insight into the thoughts and research that drove these designs, the problems the team faced along the way, and the ways they finally brought these visuals to life.

Stellaris: Infinite Frontiers novel by Steven Savile (ebook)
From best-selling author Steven Savile comes an original novel based on the science-fiction setting of Paradox's Stellaris. When the Commonwealth of Man receives proof that they are not alone in the universe, humanity is divided: should our species seek salvation in potential friends among the stars, or prepare for an inevitable war? What discoveries await the colony ship as they journey into the unknown to find the source of a mysterious alien signal? Download and read on epub, mobi (Kindle) and PDF.

Exclusive Avatar and Forum Icon Galaxy
Show your love for Stellaris on the Paradox Forums and other social networks, and Make MySpace Great Again!

Signed Wallpaper
Pay your respects to the pioneers who ventured forth into the unknown with this desktop wallpaper signed by the Stellaris development team.