I am assuming that the italicized AC modifications and the Attack Roll modifiers in the Armour section are specifcally for the Chainmail Attack Tables? I'm not sure whether the negative modifiers apply to the Wearer or the Attacker?
They can be used in the OD&D alternative combat matrix.
The penalty is applied to attacks against the one wearing the armour - this is done for systems (such as Wolves Upon the Coast) where AC is never modified due to being used in multiple rules. If your game of choice only uses AC for protection, you're fine to use the 'normal' statistics and ignore those in italics.