how is this ion engine more efficient than just squirting a small amount of pressurized gas out of the tank instead
It has to do with how quickly you can throw the propellant - how much momentum you can impart to it, which in return imparts a certain change in momentum to the rest of the satellite (delta-v). With conventional satellite propulsion, like fuel+oxidizer rockets or monopropellant thrusters, the energy available to impart that momentum is chemically based. That is, the propellants undergo a chemical reaction, get hot and/or change phase into a gas, and nozzles force that gas to exit at some velocity. Details vary with engine and nozzle design, but there are limits on how much thrust you can get each fuel type. Mass in, reaction energy, mass*velocity (momentum) out. Rocket designers measure this "efficiency" with a quantity called specific impulse (measured in units of seconds) For a given mass of fuel, you can pretty quickly calculate what the total delta-v the satellite has available to it.
Ion engines can impart much higher velocities to the "fuel"
than chemical rockets, in part because they are using electrical energy (of which there is an arbitrarily large supply) rather than whatever you can get from chemical reactions. Again, the details vary based on the design, but ion engines tend to have specific impulses much higher than chemical rockets. The actual thrust (i.e., total force) from an ion engine tends to be miniscule, but is provided very efficiently, and can be produced for days or weeks at a time.