There is a big difference between using a meme (git gud) in a post that is otherwise fully and wholly based on a valid point, and a comment such as yours which had no purpose other than to shame someone for their rank.
The Universal Rules explicitly prohibit rank shaming - and while "get good" as a post by itself would most certainly be inappropriate, using it in a context where it appears either as a) an obvious meme or b) sarcastically saying "there's better examples of the game being broken" is not inappropriate enough to merit deletion of the entire post.
I also want to take a moment here to point out that the existence of other bad posts does not give you excuse to make one yourself. Put simply, it is fully possible you and the other poster can both be wrong, and you should not use what others do as a guide for what your posts should be.
Sarcasm and vague memes such as "get good" are not prohibited on the boards - but blatant attacks such as rank shaming are. If you have any questions about this please do not hesitate to contact us in the Boards Moderation Discord server - you'll be able to speak with either the moderator who deleted your post, another member of the team, or the Heralds, volunteers who provide oversight. I would not expect your comment to be undeleted however, as I have pointed out it is inappropriate and rank shaming of any type is not welcome on the boards.