Assuming its not trivial, its hiding a useful tool from your coworkers. Why share any knowledge at all? Why mentor someone if it just gets them more work?
This isn't any different than keeping a bunch of text snippets that you use as a starting point for creating things. Text snippets may seem trivial in comparison to some tool that writes code or checks for your common errors, but when there is a bug in your text snippet, you own the bug.
Let's say he gives away his tool and all of a sudden he is the guy who has to work on it when other people have problems. Now his coworkers have more time, he has less, and he is the scapegoat for when things go wrong. Not only that but since it is not a real product of the organization, no one will recognize the time he works on it so he will have to work over time to meet his other goals.
Now if someone wants to ask for a copy of what I use, sure but they own their copy and I own mine.
Sure, there's plenty of reasons to not push every line you've written. However, the stated reason is to keep a "competitive advantage" against their coworkers.
Cause some of the tools are completely outside of the scope of work or company culture or tech stack.
We do share knowledge but not all of it is relevant, so not every little thing needs sharing. If I come to meeting and share everything in my head I might as well crab a box to stand on and preach to the clouds. I'd rather just get on with things than trying to convince people of my way of working.
Seems pretty toxic to me.