Every edition prior had a fundamental difference in that Area Direct Combat spells had to have LOS from the caster to every target in that area
Go back to earlier edition and you will see the rule structure is similar.
General LOS(A) range rules require physical line sight to all targets in the AoE. Actual straight Line of Fire was not required, you could cast the spell around corners using mirrors or reflective surfaces etc as long as you could "see" the target(s). You could for example cast LOS(A) spells by using mage sight goggles. This was the default blanket rule for all LOS(A) ranged spells (including, but not limited to, direct combat spells).
This seem to still be the intent in SR6 as well (although I agree perhaps not as clear as it was in previous editions, but this seem to be the case for most rules in SR6 to be honest).
whereas Area Indirect Combat spells only required LOS to the central point, and then targets that have no LOS from the caster are still hit.
In previous edition Indirect Combat Spells was an explicit exception to the blanket rule.
Instead of depending on a magic link and physical sight line - Indirect Combat Spells created an actual tangible physical elemental effect (such as a fireball). Since you were throwing a tangible physical effect rather than establishing a mystical link you also didn't have to have to see the "true light" of your targets.
This intention might or might not still be in SR6 as well (although this is, if possible, even more unclear and can for sure be read in more than one way if you want to do that).
SR6 p. 132 Indirect Combat Spells
Rather than having the magic do the pummeling, Indirect Combat spells create an effect that causes the damage—igniting a fireball, say, or sparking a lightning bolt.