Thank you for the post enquiring about the SoundTouch 300 Soundbar. I am sorry to hear of the confusion you have received about the HDMI cable that is included with the sound bar.
I can inform you that the HDMI cable included when you purchased the soundbar may have been version 1.4a. The soundbar does support HDMI cables of version 2.0 - which includes the 18 gigabits transfer range as you were enquiring about.
I hope this resolves the issue you were having. Please do let us know if there are any other enquiries you have regarding the SoundTouch 300 Soundbar. We look forward to hearing you from you soon!