I imagine it's similar to working groups [0], but keeping the term the same when only one person is in the working group, hence single engineer (working) group. Basically, parse `group` as in `working group` as a single term of art, rather than literally meaning a colloquial group, ie 2 or more individuals.
I have nothing against a working group that happens to be one person at the moment, but this SEG is defined by being a single person. It would cease being a SEG if it got more members. Hence don't call it a group...
[0] https://en.wikipedia.org/wiki/Working_group