This is true when all network delays between the synchronized device and the time reference are deterministic and accounted for in the configuration. The design of PTP assumes that this is the case. NTP, on the other hand, estimates the network delays to its time references.

Is there any reason to believe that PTP would be better in normal networks?

PTP is more precise so it's much harder to synchronize over long distances. Even in data centers it benefits from hop-by-hop participation from the routers involved.