The 802.11 header has 4 MAC addresses: Source Address (SA), Destination Address (DA), Transmitter Address (TA), Receiver Address (RA). The TA and RA are those of the Wifi station and SA and DA are the Ethernet addresses. This allows bridging Ethernet and Wifi interfaces and it is how many (most?) Linux-based Wifi routers work.
Yes, if you're using WDS, otherwise you only have three addresses used. Since you need the transmitting device to populate the DA, you can't just turn on bridging on a client and have everything work - you need both ends of the link to be involved.
Yes, for a wifi client (like in the article) you're right. manuel_w was talking about bridging on the AP and for that you do just enable bridging and it works fine.
