Unless they wanted to clarify that you couldn't see from outside a heavily-obscured area to inside it. Which they did. Kind of. Ish. They really didn't accomplish much with that erratum.
The errata was necessary for normal darkness to work properly. As originally written, anyone standing in normal darkness suffered from the
Blinded condition and so couldn't even see distant light sources. This would have meant (e.g.) that stars couldn't be seen at night, torches would be invisible outside of their dim light radius, etc.
@FrogReaver The basic problem is that opacity and illumination are complex topics that don't lend themselves well to succinct rules. Ergo, trying to use a close reading of the obscurement rules to divine how the
Darkness spells "actually" works isn't meaningful, because the rules don't support such a close reading. (For example, even the errataed version of the rules starts falling apart when trying to apply it literally to opaque heavy obscurement.) Instead, the DM just needs to decide if they want to treat the
Darkness spell as an opaque inkblot or instead as a transparent zone of magically induced non-magical darkness. Neither interpretation can be excluded based on the text alone, so it's simply a judgement call.
Note, however, that the transparent zone of magically induced non-magical darkness interpretation is going to produce a
ton of headaches when trying to figure out what such a zone looks like from the outside. The spell text certainly doesn't say, and trying to rely on physics is problematic, as you noted. (It's even more problematic in this case, because a physics-based approach will give entitely different answers depending on how detailed you want to get.)
The fact that the visual appearance of the spell under the "transparent zone of magically induced non-magical darkness" interpretation isn't included in the spell text is strong evidence, IMO, that the opaque ink-blot interpretation is RAI.