I believe they are referring to this EASA Explanatory Note to TCDS IM.A.120 - Boeing 737 - Issue 10:
(sorry can't post URL but it can be found in EASA document library >product certification >type certificate data sheets (tcds) )
STATEMENT OF ISSUE
The aisle stand trim switches can be used to trim the airplane throughout the flight envelope and
fully complies with the reference regulation Simulation has demonstrated that the thumb switch trim
does not have enough authority to completely trim the aircraft longitudinally in certain corners of
the flight envelope, e.g. gear up/flaps up, aft center of gravity, near Vmo/Mmo corner, and gear
down/flaps up, at speeds above 230 kts.
In those cases, longitudinal trim is achieved by using the manual stabilizer trim wheel to position
the stabilizer. The trim wheel can be used to trim the airplane throughout the entire flight envelope.
In addition, the autopilot has the authority to trim the airplane in these conditions.
The reference regulation and policy do not specify the method of trim, nor do they state that when
multiple pilot trim control paths exist that they must each independently be able to trim the airplane
throughout the flight envelope.
Boeing did not initially consider this to be a compliance issue because trim could always be
achieved, even during the conditions where use of the aisle stand trim switch was required.
Subsequent to flight testing, the FAA-TAD expressed concern with compliance to the reference
regulation based on an interpretation of the intent behind “trim”. The main issue being that
longitudinal trim cannot be achieved throughout the flight envelope using thumb switch trim only.
EASA POSITION
Boeing set the thumb switch limits in order to increase the level of safety for out-of-trim dive
characteristics (CS 25.255(a)(1)). The resulting thumb switch limits require an alternative trim
method to meet CS 25.161 trim requirements in certain corners of the operational envelope.
The need to use the trim wheel is considered unusual, as it is only required for manual flight in
those corners of the envelope.
The increased safety provided by the Boeing design limits on the thumb switches (for out-of-trim
dive characteristics) provides a compensating factor for the inability to use the thumb switches
throughout the entire flight envelope. Furthermore, the additional crew procedures and training
material will clearly explain to pilots the situations where use of the trim wheel may be needed due
to lack of trim authority with the wheel mounted switches.
The trim systems on the 737Max provide an appropriate level of safety relative to longitudinal trim
capability.