Right. But server administrators are not programmers.
Nothing (*) in the Java Language Environment (which is what the linked document covers) requires the programmer to do explicit memory management.
Optimally tuning your system requires additional knowledge beyond the language environment. That's true in every system.
CGI removed the need for developers to understand the implementation details of HTTP and TCP/IP, but if you want to tune your web-server, then you're going to need to understand those.
"Everything is a file" is all well and good for a C/Unix developer, but the system administrator needs to know the difference between the kernel parameters for TCP/IP and for Disk I/O.
SQL developers care about tables and indexes and queries and don't need to worry about physical storage or the number of execution engines, or the size of the procedure cache, but DBAs need to care about all those things.
If you somehow believed that because "Java technology completely removes the memory management load from the programmer", then no one was ever going to need to think about how much memory was used by a system built in Java, and somehow system administrators could run large Java applications without even thinking about how to tune them, then I don't know what I can say to help you.
Yes, for good or bad, Java removes memory management load from programmers - but it does not remove it from administrators, and I'm not aware of any documentation that claimed it would.
(*) Direct memory buffers (which were added long after that document was written), allow the programmer to do some memory management if so desired, but they are still not required