You are absolutely right! If I look at a random closed topic here – [discourse-docker] Support linux/arm64 platform - feature - Discourse Meta – while logged in, there is no “Reply” button, but when I log out a “Reply” button appears just after the “CLOSED 38 MINS AGO” message.
It would make more sense for no “Reply” button to appear while logged out for any closed topic. I imagine that the chance of confusing a new user (who cannot reply) by having a “Reply” button is greater than the chance of confusing an experienced user (who can reply) by not having it.