I trialed it with the new power block and my old MacAir computer. The light lit up for the few seconds that I had it plugged in.

Edited to add:
I want to be confident in suggesting this solution. I am now charging my old MacAir with the new power block and the cord that I just purchased. The computer charge has gone from 12% to 24% in the past 3 or 4 minutes.

We also have a mini power block that we purchased at the same time as the new computer. I tried it with the USB-C to MagSafe 2 cord. It did not power my old MacAir. We haven't even tried it with the new computer so I've no idea if it is an issue with the old computer or the power block itself. In looking at the schematic on the box, it appears it is meant for charging iPhones.