Microcement is an extremely robust product. That said it is only as robust as the sub-base. The sub-base needs to be structurally sound so that there is NO movement in it. Any movement will mean an increased risk of cracking to the microcement.
In terms of minimum thickness, this isn’t really the issue. A sheet of cement board is extremely structurally sound with virtually no flex and can be used as a sub-base on floors walls and worktops. Typically this comes in 6mm thickness.
So going back to the 18mm Moisture Resistant MDF, this is a perfectly good substrate or sub-base, but it will need to be “beefed up” to make sure it doesn’t move. If the MDF were to be sat on a run of kitchen units and is fully-supported, then job done!
if it is unsupported anywhere, such as a breakfast bar then some consideration should be given to reinforcing it in some way. Maybe a steel plate or an increased thickness in that area.
Get your sub-base right and the microcement will be fine.