If that is indeed the case, then what is the real reason why solar panels capture less energy at higher latitudes?
In a nutshell, air mass.
You aren't going to get a nice suntan (or a not so nice sunburn) if you sit unprotected (no sunscreen, no hat, minimal clothing) in the Sun for ten minutes just before sunset. You might well get a nice suntan (or a not so nice sunburn) if you sit unprotected in the Sun for ten minutes at solar noon at the equator.
The reason is that the radiation from the Sun has to pass through ever more atmosphere as the Sun's zenith angle increases. The zenith angle is 90° at sunrise and sunset, 0° when the Sun is directly overhead. The minimum zenith angle occurs at solar noon. At high latitudes, this is not all that low. The Sun is not directly overhead at high latitudes even at solar noon.
Even without clouds, the Earth's atmosphere is not perfectly transparent. The atmosphere reflects/bends some of the incoming sunlight away from the Earth, and also absorbs some of that incoming sunlight. As I mentioned at the start of my answer, this concept is encapsulated by the somewhat ad hoc principal of air mass. The amount of solar radiation that does reach the ground decreases with increasing solar zenith angle. Even at the northern hemisphere summer solstice, sunlight in Iceland is attenuated compared to sunlight in the tropics.