What this does is that tweets directed to @null do not appear on the tweeted user's timeline (i.e. doesn't show up on the list of tweets they made). Sometimes people use it when writing a memo to themselves but don't want it to clog someone else's timeline. Other times it's used simply to tweet crap they just don't want others to see, which is essentially talking to themselves.
However, it's possible for a third party to search or follow "@null" to reveal what someone has been saying behind the curtains, which is probably what Hacchan did here.
Why did I get hit there, I wonder...... I let Admiral down......Sigh...... Even though it's been so long, I'm such a useless person.You can't grow up on your own. Admiral too was doubting his own leadership. You are the leader of the 1st Submarine Fleet, and also Admiral's secretary ship. You two are alike. Be honest and compliment each other's weaknesses.I-8How is your injury, Imuya-san?I-8Thanks. I'm going to the command office a bit.Iku-nanooooI-8Cockroach came out. GYAAAAHHHHHH!!!!!Ah, so mutual followers can't see tweets if you use this as a header. You really are more knowledgeable than me about this kind of stuff, Hacchan...... So I guess you saw that stuff. This is embarrassing.