I have zip ties all over my engine wiring, never had one melt.
From zip ties manufacturer:
Ties withstand temperatures to 185°F (85°C) and have a melting point of 495°F (257°C); except for barbed design cable ties which withstand temperatures to 167°F (75°C) and have a melting point of 425°F (218°C). All ties remain flexible to –40°F (–40°C).
I certainly hope your engine compartment isn't seeing 495 degrees
.