I can easily prove that they weren't serious at least Luffy wasn't. There are deferent levels to bazooka, the further Luffy stretches the stronger the thecnique. For example look at the bazooka Luffy used against Wapol and the one he used against Zoro...
In case of Wapol Luffy stretched as far as to the end of the island and the thecnique was so strong that it sended Waplo flying to the another island.
While in case of Zoro he stretched only for few meters, he didn't even have the place to stretch fully you can clearly see the building behind Luffy blocking him, if Luffy used full powered bazooka Zoro would have been oneshoted or send flying.