NAC controls network access, ensuring secure entry. SIEM monitors and responds to security incidents through log analysis. Both are crucial: NAC for access control, SIEM for incident detection and response, enhancing overall cybersecurity.
I think, Prem nailed it pretty well. Just to have another point of view for this:
NAC is in the area of prevention (disallowing the "wrong" devices to connect to the network at all) and a SIEM is in the area of detection.
So, with NAC, you can prevent the initial access, a SIEM won't help here. If an attacker has access already, however, a NAC won't help, but a SIEM could uncovering the attacker's traces.