This is just a theory, so don't get all hung up on it.
In short, the reason I think they bear so much resemblance to MBTT descriptions is because they're derived from them. The only reason people like Pheadrus think MBTT types are the same as their socionics 'equivalent' are because they are written from an MBTT perspective. Put it this way:
*A socionists comes along*
"Ooh, this LSE has the same functional ordering as the ESTJ in MBTT. Hence, they must be the same thing! I will write a description of the LSE which bears likeness to the ESTJ."
What the socionist should be thinking is this:
"LSEs may have the same functional ordering as the ESTJ, but that doesn't necessarily mean they are the same kinds of people. Hence, we will write about the LSE from a socionics perspective; obviously their ego is Te and Si blocked together. So, let's talk about how said Te and Si influence the character."