As far as I know, this site suppresses all alignment features that would allow text to be centralised. You could do something close to it using code blocks, but it's not really the same.
This text is (somewhat) centrally aligned.
Typing four spaces before a line of text creates the code block, and then adding more spaces afterward pushes the text along. The downfall of this method is obviously that the text is pre-formatted, but also that it won't appear centrally aligned on devices with a low screen width (e.g. most phones).
Presumably, this works the way it does because Pokemaster set it as such. I can only speak for myself, but it looks like he wants to keep all posts left-aligned.