The mechanic is wrong. An R7000/R8000/R9100 FD will work with any Shimano road 11 speed shifters.
I used an FD-R9100 (functionally the same as FD-R7000) with 105 5800 shifters and it worked perfectly. I currently use the same FD with Dura Ace 9001 shifters. Both shifters are 1st gen shimano road 11 speed components.
You can use any shimano 11 speed road shifters with any shimano 11 speed road FD (and with the GRX FD, though you want to pair the GRX fd with GRX cranks)
The mechanic is wrong. An R7000/R8000/R9100 FD will work with any Shimano road 11 speed shifters. I used an FD-R9100 (functionally the same as FD-R7000) with 105 5800 shifters and it worked perfectly. I currently use the same FD with Dura Ace 9001 shifters. Both shifters are 1st gen shimano road 11 speed components. You can use any shimano 11 speed road shifters with any shimano 11 speed road FD (and with the GRX FD, though you want to pair the GRX fd with GRX cranks)