I'm curious, do you take "the sins of all mankind" to mean that Heidleberg does not teach particular and definite atonement? I've not read the HC, so I can't speak to what it does or doesn't teach, but I hold an alternitive view of particular atonement, that of particular redemption. I believe that Christ died to atone for the sins of the world, but that atonement is only effective for those He redeemed. From the quoted Question, I don't see that HC is teaching against particular atonement, rather I see that it teaches the fullness of what Christ suffered on the cross.

In Christ,
Matthew

"Those who go down to the Sea in Ships
Who do Business on great Waters;
They have seen the Works of the LORD,
And His Wonders in the Deep."
-Ps. 107:23-24