You mentioned that your thwn is only in conduit outside......it must be in conduit the entire run from panel to receptacle. Maybe I misunderstood but that seems to be what you are saying.
The 'ground' your talking about is the equipment ground. It starts at the panel and continues to the motor.....end of story. It facilitates the current path for any fault to ground (metal) in the motor branch circuit. This is so enough current will flow to trip out the gfci breaker on any overcurrent fault.
The motor, if it isn't double insulated, will have a bonding lug. You connect to that lug with a #8 copper bonding wire. This wire then continues to all metal parts of the pool structure plus accessories like metal ladders and anything within 5 feet of the waters edge that is metal. This 'grid', bonding all metal, is the equipotential bonding grid and prevents changes of potential between the metal around your pool. This helps prevent shock hazards that are created from external sources This is probably the other ground that the electricians are telling you about. It just isn't a ground like the green in the motor branch circuit wiring. It doesn't ground anything it merely is a bonding wire joining all metal and metal equipment associated with the pool.
If the motor is double insulated then there will not be a bonding lug present, however you still must locate a #8 copper close to the motor to facilitate the replacement of the double insulated motor with one that is not double insulated.
So two things at the motor....the equipment ground of the cord and plug terminates (ends) on the motor wiring compartment green grounding screw. Nothing else continues on from that green screw. The #8 bonding wire is connected to the lug on the outside frame.......your done.
The attached image is for an inground pool but gives you the general idea for the joining of metal around and part of the pool. The second image is an above ground pool pump. These show what Speedy was distinguishing in post #2...bonding vs grounding.
Above ground pool pump (hardwired). The
equipment ground is inside the flex weathertite conduit and terminates on the green screw inside the motor wiring compartment. Also the
bonding wire is terminated to the bonding lug as shown.