I had a look at the map provided on the UNESCO website and must say I do not understand what I am looking at.
Yes - it is a mess with its lack of reference points!
First - it is oriented
with North to the right!! (see "Compass point" at top right)
If you look at it in that orientation then there is a long "hippodrome" shaped feature with "rounded ends" at the top centre - ie the Western side of Zacatecas. The red boundary line goes just "above" i.e to the western edge of this feature.
If you then look at Google maps
for the city centre (correctly oriented N/S) it will be seen that this feature is La Alameda park running SE/NW alongside Av Torrejon. The Cathedral is pretty well central on the UNESCO map - the large rectangle surrounded by streets
It will be seen that El Eden is slightly further NNW than the "Hippodromic" Alameda OUTSIDE the core area. Torrejon continues NW until it circles around Biblioteca Publica with El Eden to its NE. El Eden is situated at the very top of the UNESCO map to the right of the sinuous Paseo Diaz Ordaz which runs along the bottom of the hill - clearly outside the red line area.
With its inscription as early as 1993, the definition of "Historic Centre" for Zacatecas was rather stricter than I suspect it might be nowadays! And it only has the single location .