I'm pretty sure the 11- 18 mm Sony is compatible with the A300 since that camera body uses a cropped sensor, not a full-sized one.
the entire Sony alpha range have cropped sensors, except the professional A900. Hence, all these alphas, except the A900, should work fine with DT lenses, which include the 11- 18mm
You can read up on the advantages of having a cropped sensor as opposed to having a full size one on the net. Another thing, cameras with cropped sensors can be used with lenses designed for full-frame cameras. But lenses designed for cropped sensors are not compatible with full framed cameras, for the reasons stated by Sony themselves, as you've already read.
It's also important to note that its generally more advised to invest in lenses for full framed sensors, since, you never know when somewhere down the line, you might upgrade from cropped to full size sensor cameras.
Try watching the review by Cameralabs on the 11- 18mm. Gordon uses it on a cropped sensor camera.
Hope this helped!