When you puncture a hole in the thermos, it is no longer a closed system. The high rate of velocity ejected from the bottom of the punctured thermos is due to the relatively low atmospheric pressure from the air outside.
Also, I think you are confusing two different types of pressure. Dynamic pressure (what you are referring to) is the pressure exerted by the fluid during movement. So here a faster moving fluid will have lower pressure. Hydrostatic pressure is the pressure exterted by fluid at certain depth. So at a position that is deeper in a fluid will have a higher pressure.
So back to your example, in an intact thermos, hydrostatic pressure at the bottom of the thermos will be high. Once you puncture a hole at the bottom, the fluid will experience a pressure difference between inside the bottom and the outside atmosphere. Fluid will flow in any instances where you have a difference in pressure. The hydrostatic pressure of the fluid is greater than the atmospheric pressure, hence fluid is ejected from the bottom of the thermos out into the surrounding at great speed. The ejected fluid itself will have a much lower pressure compared to the one inside the thermos.
You can reason with bernoulli's equation, but I prefer picturing it conceptually (without all the crazy formulas and numbers). Hope that helps..
