Yes that's correct. There are many occasions where I/we need to use up the entire allowable weight to achieve an operational requirement even if that means operating at the maximum allowed limit or on the edge of the envelope, we understand this and even acknowledge this in our risk assessments for AOC flights. This is quite normal. So if the RFM Maximum Takeoff Mass is say 2850kg, the RFM allows an All Up Mass of 2850kg and this is within limits - even though it's on the limit, it's not outside limits as the warning suggests.

The issue is, it shouldn't raise a warning when on the limit, only if exceeded. 

Also, some operators use SD for W&B and therefore we have to save/print the briefing pack for documentary evidence. This warning is shown in the briefing pack and can cause a bit of a fuss during subsequent audits - this can be justified if the AUM is on the horizontal part of the CG envelope where the MTOM is obvious, however on the sloping parts of the CG envelope its a bit more difficult.

Hope this helps.