Speaking in general, Portmaster is a privacy tool, focusing on privacy features like DNS encryption or blocking tracking requests.
Anti-Virus systems, such as Windows Defender, on the other hand are security tools. As such, they focus on security features like malware detection, which Portmaster simply does not have.
Please note the important difference:
Anti-virus systems sometimes also _do privacy_, but they _focus on security_.
While Portmaster also _does security_, as privacy _depends_ on security, it _focuses on privacy_.
That is why we do not recommend deactivating Anti-Virus systems. We recommend having both have a security and privacy setup. Without security your perceived privacy can quickly become non-existent.
However, research if the Anti-Virus you are using is really as good as it claims. Many security researchers have come out in the recent years, declaring that Windows Defender is good enough for most use cases and quite some other Anti-Virus systems are doing weird stuff.
Naomi Brockwell also has a great video going into the details and nuances about the topic of Antivirus.