Immutable String Buffers

Why is this telling me to concatenate small strings to the ends of large ones, but not in reverse, and then concatenate all remaining strings in the table?
http://www.lua.org/pil/11.6.html

Wouldn't it just be easier to put all the strings in a table then concatenate them all at once, as in the following?

function filetostring(file)
    local t == {}
    for line in io.lines(file)
        if line == "" then
            break
        end
        table.insert(t, line .. '\n')
   return table.concatenate(x)
end

I'm confused as fuck.

Because otherwise you end up allocating and copying excessively. The best thing you can possibly do is to use a StringBuffer/StringBuilder/whatever-the-fuck-it's-called object that tries to minimize extra copying and allocation.