what leads you to make this claim?

nothing has been changed in jack1 transport in a VERY LONG time.

     http://jackaudio.org/files/docs/html/transport-design.html

the docs say:

      The engine handles polling of slow-sync clients. When someone calls jack_transport_start(), the engine resets the poll bits and changes to a new state, JackTransportStarting. The sync_callback function for each slow-sync client will be invoked in the JACK process thread while the transport is starting. If it has not already done so, the client needs to initiate a seek to reach the starting position. The sync_callback returns false until the seek completes and the client is ready to play. When all slow-sync clients are ready, the state changes to JackTransportRolling.


and also:


    These request a new transport position. They can be called at any time by any client. Even the timebase master must use them. If the request is valid, it goes into effect in two process cycles. If there are slow-sync clients and the transport is already rolling, it will enter the JackTransportStarting state and begin invoking their sync_callbacks until ready.