ZAP is an independent Open Source project controlled by the ZAP Core Team and supported by Checkmarx.
It is known as “ZAP by Checkmarx” because 3 of the Core Team are employed by Checkmarx to work on ZAP, for more details see the blog post ZAP Has Joined Forces With Checkmarx.
The ZAP Team regrettably had to take ZAP out of OWASP in September 2023 in order to obtain enough funding to keep maintaining and improving the software.
The ZAP Team are still OWASP supporters and some of us are still OWASP Project leaders.