In the end, what matters most is Guardians of the Galaxy 2 being an excellent film that entertains people in the theater.
These days, it’s become common for theatrical previews to include spoilers, unfortunately diminishing the impact of seeing the final product on-screen for the first time.
The short trailer tease is a perfect illustration of the balance Disney/Marvel needs to find when promoting any of their films.
This ultimately shouldn’t be the biggest deal, as this was just an amusing moment to highlight the now-famous Guardians brand of humor.