I think it's more likely that it's caused by lack of read buffering in early implementations. The function that reads an SMTP command just reads whatever it can

I think it's more likely that it's caused by lack of read buffering in early implementations. The function that reads an SMTP command just reads whatever it can get from the kernel. If the connection is half-duplex as expected (following the one command, one response pattern), there will only ever be one SMTP command in the buffer after the read returns. With pipelining, that is no longer true, and the implementer has to figure out how to do proper buffer management. (I don't why this is not mentioned in RFC 2920.)