I'm quite used to transposing music to where it fits my instrument, even though I know some people don't approve of this. I think what is important to know when doing this is what character is implied by using a certain signature. Since I don't have a copy of Mattheson's Neueröffnetes Orchestre I looked for the quote and found this page that quotes Mattheson directly on what they mean. Here is a translation to English by Babelfish

In this last quote Mattheson's Der vollkommene Capellmeister, we go back to religion. There are just a few things that a good capellmeister cannot afford not to know:

"...to God even a thousand years is like the day which passed yesterday, and the angels' days are years, as is known by the theologians." Mattheson quotes Luther here from Daniel.

But let's return to the origin of music: "Angels, though they are spirits, can assume bodily for, can use instruments and appear in flesh and blood, as Michael did, as often as they want. And since in the hereafter we redeemed men like them will be in body and soul, it is easy to divine what a magnificent quality the harmony of these heavenly musicians must have had.

Thus it is not true that vocal music is actually and originally older than instrumental: for all sort of instruments are also ascribed to the angels and saints in the Bible, especially harps and trombones, as string and brass instruments, and they certainly played just as well as they sange before Adam was created."