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You are correct if you assume a series wound motor that is internally connected and only has 2 external terminal. That motor cannot be reversed simply by reversing the motor polarity at the external terminals.
For reversing to occur in a series or shunt wired motor you must have 4 external terminals. You reverse the series motor by reversing the field preferably. You reverse the shunt motor by reversing the shunt winding preferably. How can you tell the difference between a shunt or series motor when they both have 4 poles? The field on a shunt motor is only sized for providing the field current - much smaller the the armature current. A series motor must carry full current through both the field winding and the armature winding. Take some resistance measurement of the armature and the field windings of both types of motor that are similarly sized and you will immediately see the difference between the 2 of them.
The only Dc motor I can think of that can be reversed only having 2 wires leading to it is a specially prepared dc shunt motor that has its shunt winding internally fed from a diode bridge so that no matter which polarity you feed to the motor the diode directs the power to the shunt winding with the same polarity even though the power is switched going to the armature.
I know the simplified drawing I had a link too did not provide the complete picture. I will try to find my drawings from when I built an IGBT controller that was controlled by a 555 timer and was connected to 240v dc - 5 full size 48 volt forktruck battery packs. It had 16 - 300 amp Darlingtons set up in an H-bridge. It was used to power a 150 hp DC traction motor for switching trains inside of a Steel Mill I worked at.
It was not pretty, it was not ready to be driven on the street safely, in fact it was a bit scary - but it worked for 4 years that I know of and only failed one time taking out 4 IGBT's - they turned into slag.
But it did shift trains full of steel weighing hundreds of tons.
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