I wouldn't make any assumptions about why you were sent the video if there was no commentary included. If you're curious, ask. He may have wanted to give you an idea of what the difficulty level they are capable of, or the tone quality of their particular instruments, in case that would be helpful, or he may have just been being self-promotional and hoping you would share the video with your music-loving friends to help generate excitement for their performance of your work and other performances in general. There is a lot of back-scratching between various music contacts in my world. Many of the people I sing with are in multiple groups, so we spend a lot of time trying to sell tickets to each other and to each other's friends. You may now be on the long list of people that he sends an email to whenever there is something to announce or promote the group.
That said, I think your piece is a nice length. If you want to extend it even further, think of smaller and larger structures, (symphonies are made up of movements, movements are made up of A and B sections, sections are made up of statements of the theme in various ways), and giving different players moments to shine individually or as duos and trios against the larger texture. You're doing a lot of that already, but if you get stuck you can always cut and paste the structure from a piece of music by someone else that is a good length and that you enjoy. Break it down, measure by measure, who has the lead, who is supporting, how many times does the theme repeat and what changes when it does. (: I'm playing with the same set problem right now, so I feel your angst! Good luck! This sounds good!