Hello and hope this could help you! :)
To prevent the STP flapping caused by the device restart and peer-link faults and to ensure switching performance, you are advised to set the delay in reporting the Up event to be at least 30s on the M-LAG interface, peer-link interface and other service interfaces. By default, the delay for an M-LAG interface to report the Up state is 120s in V200R005C00 and earlier versions and 240s in V200R005C10 and later versions.
So the 240s that you've checked in the product documentation is the spec in V200R005C10 and later versions and the 120s in the M-LAG best practise is the spec in V200R005C00 and earlier versions.