This is true. However, the SDK for the M3E is version 5. The SDK for previous DJI drones is version 4. SDK v5 is not compatible with SDK v4 meaning both SDKs cannot be used in the same piece of software.
Had DJI supported the M3E in SDK v4, Litchi would have to make “minimal” changes to their existing software to support the M3E, which would not require a long period of time. But, since software developers have to now use a totally different SDK, they will have to support the M3E with a totally new application. As a result, supporting the M3E will take much longer.