I think the 2'-6" interpretation of the code regarding "engineered" projects may be a local one or out-dated. When it comes to requirements, a code jurisdiction can adopt any reputable industry standard or model code.
An engineered wall is not just anything over 2'-6". A code can specify the maximum height of an "un-engineered" wall and it is usually in the range of 4' or 5'.
The engineered footing can be either a concrete footing (width, depth, thickness and reinforcement) OR it can a system of different solid layers of base and sub-base and compaction layers with no concrete footings.
I have seem many engineered retaining walls using SRW(sentimental retaining wall) units that were between nothing and 30' high high that had no concrete footings or mortar (bed and head joints) and just geo-grid soil reinforcement. The most extreme wall I have seen was on highway that was about 600' long and was between nothing and 40' high. These were concrete block sized units and not massive blocks measured in feet mad from come back concrete.
The 2'-6" limitation seems a little archaic and limiting to owners, designers and responsible contractors. The amount of unit being made available without any support or background makes thing a little bit "clouded" for the public. I "garden walls" should be limited to walls retaining a reasonably sized garden that is used as a garden and has controlled drainage and a reasonably level surface.
Dick