tl;dr: USB Power Delivery doesn't require the CC lines of a USB Type-C cable, it can also communicate over the power line.
When most people think of USB Power Delivery, they probably think of two USB Type-C devices talking to each other. However, the Power Delivery standard is actually older than that—revision 1 of USB Power Delivery didn't even mention USB Type-C!
Power Delivery's negotiation (the process where the charger and device agree on the voltage to use) happens over the physical layer, and there are two options for the physical layer:
The first option is, again, what most people think of, and it's using the CC pins of a USB Type-C cable, in yellow below:
The second option (which was actually the only option until Type-C came around) is to use the Vbus line of a non-Type-C cable.
Think about if you were designing this: it has to work with USB 2.0, which only has Vbus, D+, D-, and Ground. Normal USB data communication needs to still work, so you can't use D+/D-...the only option left is Vbus.
But wait! How can you communicate over a power line? We can check the official Power Delivery specification:
Apparently, this version of USB-PD used Binary Frequency Shift Keying (BFSK) to modulate the packets over the voltage line. The exact details are probably out of the scope of this post, but if you're interested, I got that screenshot from page 116 of Revision 2 of the USB-PD spec (linked at end).
That explains how USB Power Delivery used to work, but then how is it still compatible with a modern Type-C device, like your laptop? Well, the spec has an answer for that too:
(note that here, "BMC" refers to "Biphase Mark Coding"—that's the first option I mentioned before, using the CC lines. The screenshot is page 138 of Revision 2.)
This basically means that, if a device supports communication over CC, it's also able to support the old-fashioned Vbus method for compatibility reasons. They even mention the scenario you're describing: "Note that any system utilizing the USB Type-C connector can see BFSK signaling on Vbus when a suitable USB Type-A/B to USB Type-C adapter is used."
It's not a requirement, so it's possible you could find a device that doesn't support this. It's also not allowed to work the other way around: if a device has a Type-C port, and it supports the BFSK method, then it must also have support for the CC lines.
If you're interested, you can download the official spec from the USB-IF website. For whatever reason, the download seems to only have Revisions 2.0 and 3.0, and I can't find the original Revision 1.0. Revision 2.0 describes both methods though, and is what I used for everything I referenced here.