On the AE-1, full-aperture metering works because compatible FD lenses mechanically tell the camera their maximum aperture.

With the Canon FD mount, there are communication linkages between lens and body, including:

• a lever that tells the camera whether the lens is at full aperture or stepped down
• a pin that tells the camera what the lens’s full aperture is

So the camera does not just measure raw light and guess. It measures the scene with the lens wide open, but it also knows how wide open that lens is, allowing it to calculate the correct taking aperture for the selected ISO and shutter speed.

That means the same scene should meter correctly with different FD lenses, even if one is f/1.4 and another is f/2.8.

FL lenses are different: they do not provide the full-aperture signal pin, so the AE-1 cannot fully compensate in the same way. With FL lenses, the meter generally cannot directly indicate the final aperture in full-aperture mode; instead, you typically meter by stop-down behavior or by relative adjustment.

So your concern is valid in principle, but with FD lenses the camera does know the lens maximum aperture.