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.

>>6
That's what I thought, but then why is "Programming in Lua", by the Chief Architect of the language, telling me to do something less efficient. Is it just to prove a concept? It reads as if he thinks it's faster.