It just seems like a bad tactic. For example, if the US gives Ukraine some software that helps them fight Russia, it's likely tactically advantageous (to Ukraine) if Russia doesn't have the source code.
Of course, it doesn't mean Russia couldn't do some reverse engineering to some extent. But that takes time, and likely wouldn't be as complete/thorough as just handing them the source code.
This kinda sounds similar to how things worked before GitHub, when people just emailed their git patches. Some OSS projects still work that way.