Can I expect issues to be addressed in version 12.0.0?

12.0 Provides the latest advanced features at the cost of slightly higher risk on stability. If your client requires some of the features only available in 12.0, go for the latest. In other cases, as a default choice, you should generally recommend your clients to pick the latest stable release (11.5.3 HF2 as of today).

Your question: I am wondering if I understand the following correctly. Does the bold text imply that product defects, security fixed, diagnostics and support improvements, and new support wont be addressed in 12.0.0?

Not entirely accurate. Any issues, such as software bugs and vulnerabilities will be addressed in case of short-term releases, however, the duration of continuous development and support is generally less than in case of long-term releases (n.1.x). As of today, end of technical support date is not stated for 12.0, but you can safely expect to receive technical support for 12.0 for at least until 2020.