That is probably because it counts as magic damage and thus takes your magic accuracy in account, which is obviously very low in melee gear
I went out and tested the above quote using magic gear and melee gear on my good friend Tgod Bro. I used thirty special attacks on him while he was wearing the following gear:
I tested magic armor wearing this gear:
I highlighted the magic accuracy of this setup, it's severely low in comparison to melee even though korasi deals "Magic damage". The accuracy should also be severely heightened by Leo's lower magic defense in his melee armor, but this is not the case.
These are the results of thirty special attacks used with a magic potion, a super strength potion, and magic piety:
1: 0
2: 0
3: 0
4: 32
5: 0
6: 44
7: 0
8: 0
9: 0
10:0
11:5
12:0
13:0
14:37
15:0
16:0
17:0
18:36
19:0
20:0
21:0
22:30
23:0
24:19
25:20
26:0
27:0
28:0
29:42
30:0
Pretty pathetic, it hardly hit at all despite my magic accuracy and Leo's magic defense.
I tested melee armor wearing this gear:
I highlighted the melee accuracy with this setup, it's WAY higher than the magic setup, and trumps it by a whopping 106 accuracy.
Here are the results of thirty special attacks using a super strength potion, super attack potion, and melee piety:
1: 32
2: 19
3: 62
4: 54
5: 0
6: 58
7: 62
8: 19
9: 0
10:0
11:0
12:17
13:0
14:0
15:1
16:42
17:0
18:0
19:31
20:0
21:42
22:44
23:0
24:12
25:41
26:0
27:58
28:65
29:26
30:16
Far better results than the magic armor. It appears melee is far superior to magic armor while using a korasi, even though it's special attack deals supposed magic damage.
TLDR;
DON'T USE MAGIC GEAR WITH A KORASI, NO MATTER WHAT MARY TELLS YOU!!!