In fact, I don't even think the client program for Bazel is written in Java, but C++, and the Java daemon it talks to is started when you first attach to a Bazel workspace and it persists, so subsequent interactions are very fast. Just running `bazel` randomly is not truly indicative of what using it feels like, because that's not what people actually use it for. People use it inside a build workspace, that is the case that matters.
Beyond that, other build systems like Buck2 (Rust) also use the client-daemon architecture for a number of other reasons, including the fact that actually-really-large build graphs are far too large to rebuild on every invocation, so the daemon is necessary to keep the build graph and interactively analyze and incrementally invalidate it on demand. Doesn't matter if it's Rust or Java, these things are architectural. That's also one of the points of the article, of course, and why they're theorizing about server-side analysis caches and graphs, etc.
This all indicates to me that the people designing these systems actually do care about interactivity of their tool. It is not a matter of "java startup slow" to use a client-server design, though it certainly is known to help.