Bludgeoning clearly doesn't have to be rapid, since it is used to represent plenty of slower crushing damage. Every constrict attack is bludgeoning and so is the rug of smoethering's smother attack, the rolling sphere trap in the DMG does bludgeoning when it runs someone over (it has to enter the creature's space to do damage, it knocks them prone, and it's going way too slowly for it to be the impact), Bigby's hand does bludgeoning when you use it to crush someone with grasping hand, and Maximilian's earthen grasp also does bludgeoning while restraining. So given the significant precedent, it seems far more reasonable to describe the tennis ball as having a higher AC than the skull, and maybe resistance to bludgeoning damage. You can absolutely still do bludgeoning damage to it.
So based on that, bludgeoning damage is just any physical force applied in a way that won't typically pierce or cut a target; I can cut a block of soft butter by pressing a rolling pin through it, but edge cases (no pun intended) like aren't really useful for general purpose rules, so we say a rolling pin does bludgeoning damage when you hit me with it. We can apply this to the oil jet just the same: is it focussed enough to cut in to most targets? Piercing damage, maybe with some necrotic damage if it's some nasty oil that would be very bad to have getting inside you like that, as is often a complicating factor in high pressure fluid injuries. If it's less focussed than that but still hitting with enough force to hurt you, then we say it's bludgeoning, just like a marid's water jet attack or the tidal wave spell. Either way it's not doing some mysterious force damage that doesn't have obvious parallels in the other mundane damage types.