I’m not sure if the that sentence means that the motor can’t draw anymore than 1.5 amps because of the current capacity of the power source (battery) itself is limited, if there is another reason due to the rating of the motor or if there are other circuit considerations here. I’ll try to answer this regardless.
Think of amps as volume and voltage as pressure. Think of a water line for example: your water pressure may be very high but if the piping diameter is too small or if the volume capacity of the source is too small then regardless of the high pressure the water supply may not be ample enough to perform all of the tasks you want it to. You need both of the correct pressure and the volume capacity to perform the proper work.
Don’t forget that a motor is an inductive load and not a resistive load. Inductive loads will produce a massive current (amp) surge when initially energized, then after the start-up period the amperage will level off to its normal current. This current will vary depending upon the horsepower (one hp is equal to 746 watts in a motor). Ohm’s Law formulas for determining the current draw of a load will not be accurate for inductive loads as they would be for resistive loads. A great example of this is a typical three phase step up/down transformer. The ohms you would measure on the transformer windings via a multimeter can be as low as 1 ohm, and you can have 480 volts feeding the coils.