I’ve just noticed that on closed topics the Reply button still appears. I’m on 2.8.0.beta7.
I’ve checked while logged in as admin, and logged out, and the Reply button is still there. Then I used a different browser, cleared all cookies, and it’s the same. I’ve logged in as a TL1 user and the Reply button disappears but when I log out again it’s there again.
It seems likely that Discourse has a clever way of knowing the forum administrator has logged in from this computer… but, if not, is there some setting I should change?