Meh, besides the reveal & it's impact on Abe, the episode was pretty weak. Plus, the walkers' skulls would have to be the consistency of warm jello for a high pressure hose to rip through them like that. I don't even think their bones could support the weight of a human body if they were that soft
That's almost a trope we have to roll with when it comes to this series and genre. The episode where Bob died Tyrese uses Sasha's pocket-knife to stab into Bob's brain, through his temple. This is when Bob has *just* died so we can't hand-wave any decomposition. Stabbing through the skull with a pocket-knife I can't imagine would be very easy.
Here, we are dealing with walkers who've been rotting for the better part of 2 years, and though we've been told the decomposition is very slowed with the walkers compared to a "normal" corpse, it's still occurring. (Though suggest maybe it's stayed off if the walkers stay "fed.") Maybe these walkers were trapped in the firehouse for a long enough time. "unfed", they were sufficiently decomposed for the pressure of the fire-hose to pose a "lethal" threat to them. Their body structure being "strong" enough for them to walk and stand but only *just* the hose was enough to push them over the line.
Think of a very thin cracker, cookie, sheet of candy, ice or something along those lines. You handle it delicately enough and it remains intact. You might be able to balance it on something or balance it upright and so long as the forces acting on it to keep it standing/balanced doesn't surpass it's limits it'll remain intact. Test those limits and it crumbles/shatters.
The pressure on those hoses is, however, very, very, very strong. Though I'm sure it requires the engine of the truck running in order to operate the pumps that provide the pressure and means to expel the water through the water-turret.