The condition that the matrix \(\delta_E\) is strictly upper triangular is to ensure that the sum in the Maurer-Cartan relation converges. One can also ask that there exists a filtration on \(E\), the formal direct sum of shifts of objects, and that the differential \(\delta_E\) respects the filtration (see Section 31 of [Sei08]). From this perspective, the twisted complex looks more like a formal deformation of the direct sum.
References
[Sei08]
Paul Seidel.
Fukaya categories and Picard-Lefschetz theory, volume 10.
European Mathematical Society, 2008.