No, that’s not correct. The FMC has no concept of your estimated takeoff weight nor does it attempt to predict it. What it copies now is exactly what it copies in the real 787 EFB.
Since this is a simulation of the actual 787 EFB, and in real life you can’t change the weight and balance of your aircraft via this EFB, this functionality does not appear there. However, in the settings page on the FMC there is a load page for properly loading the aircraft that does not require the sim’s weight and balance panel.
The EFB page is for takeoff computations (and is identical to the real aircraft’s), not specifically for data entry (although you can enter the data there as well). There is no other location where you can get confirmation of safe runway lengths or get a computed and correct derate for the selected runway and length.
If you have no interest in using the correct derates for the field length or validating field length overall, checking if the flaps setting is valid, getting your V speeds, etc, then you may find the EFB takeoff performance page not useful for your particular use case, but that’s what real world 787 pilots use it for.