Its not that weird. DSM2 is not longer legal for new sales in the EU as of last year 2015. I'm sure Spektrum were anticipating that for some time. The DX4e, DX5e and DX6i supposedly all shared the same 6CH module but there may be variations in the firmware used.
DSMX uses the same binary signalling as DSM2 but it uses a different RF channel management scheme. The DSM* protocol and variant used is specified in the bind message. I very much doubt that both channel management schemes are in operation at the same time so, assuming that, DSMX is technically not backward compatible with DSM2, but DSMX equipped TXs and RXs have been able to do either. Except now in the EU they can't!
I'm unclear how the transmitter decides that the Receiver is DSMX and not DSM2. There are a number of variants of each. I think there is a short range RF transmitted ACK from the RX, a kind of nacent telemetry signal, that indicates the RX is DSMX or DSM2. This discussion about binding satellite receivers
supports that notion.
OpenTX sends a DSM binary stream to the DIY module for the channel "transfer" packets. It does not use PPM. I'm not sure how binding works in this case. It is possible it is all done by the Spektrum TX module but it has to be initiated by OpenTX - there is a soft BIND "button" and a choice between LP45, DSM2 and DSMX. LP45 is the low power "France" DSM2 mode. DSM2 and DSMX are presumably their respective modes. Possibly the DSMX selection does not implement fall back binding to DSM2. Possibly they all use the default lowest common denominator variant and just assume the RX can speak it without bothering to listen for an ACK. Have to poke around in OpenTX code to find out.