The Nahema can't be a new construction.
The Sekhmet is based off of the Nahema and designed by HoL. This is stated in its tech description.
HoL was wiped out in Operation Templar before FS2. This is based off Operation Templar, which is canon.
Therefore, Nahemas must have been around before FS2.
The only time Shivans ever entered GTVA space before FS2 was during the Great War.
Nahemas were sighted before FS2.
Therefore, Nahemas must have appeared, perhaps very very rarely, in the Great War so that the HoL would have had a chance to see it, and then design a bomber based off of it.
Since one FS2 Shivan craft has technically been around since FS1 (just not sighted by the player), it is possible that the FS2 Shivan craft such as the Rakshasa would have been around at that time too. They probably weren't part of the Lucifer fleet though.