...Zamak which has a higher tensile strength than steel.
As mentioned, the zamak receiver cover is not a stressed part, so its strength is not an issue.
However, the tensile strength of zamak is not higher than that of steel.
There are a number of zamak alloys, with tensile strengths ranging from 268MPa to 397MPa. Zamak 5 tensile strength is 331MPa.
Low-carbon mild steel alloys (on the very low end of steels in terms of tensile strength) will have tensile strengths in the range of 370MPa to 410MPa.
The steels typically used in firearms have tensile strengths in the range of 590MPa to 830MPa.
In other words, the strongest zamak alloy can match or exceed the strength of the weakest steels but can not begin to approach the tensile strength of a good quality steel. The tensile strength of zamak 5 doesn't even match the weakest steels.