Is 'свидетельствовавшему о том, что...' a standard phrase of some sort?
Yup. It's "testifying to the cooks in the centuries to have started preparing dinner".
I hope it makes sense in English.

кентурия (центурия) is a Latin military unit consisting of 100 men.
https://en.wikipedia.org/wiki/Centuria

верхняя площадка сада looks like an UPPER LANDING of a garden (for step-down/multi-layered/staircase-like ones)
http://www.acrossrome.com/wp-content/gallery/13/06.jpg