In fact, I feel it is easier to call everything posts. Create a new post or reply to an existing post. To me, that is way easier than having official words.
I use it interchangeably. I sometimes call them topics and sometimes threads. However, replies are always posts for me, I don't use the word comment when I want to refer to replies on forum threads or topics.
I mainly use threads or discussions, not so much topics. I think the term "Topics" was much more popular years ago, but have seen most forums transition into either Threads or discussions
I have always been someone that has used both. Usually, it goes by what the forum I am posting on goes by so for example if they call them threads I will call them threads and if they call them topics I will call them topics to save confusion.