I'm pretty sure Viel explicitly states that the promise for the Lightborn not to take the field of battle is separate from the covenant. At least that is what I understood from her discussion with Thoromarth in Chapter 10. Here are some selected bits from that conversation: "But you know that the Lightborn do not use their arts in war... The Covenant is not the same thing as that promise... The Covenant is our pledge that we will never draw so much power from the land that it sickens and dies... So we wrap ourselves in custom and let the lightless think it is a vow."
So in conclusion I think the answer is that neither Mosirinde's Covenant nor Arilcarion's Art of War that prevents the Lightborn from taking the field of battle but rather a combination of fear, misunderstanding and custom that do so.
Mosirinde's Covenant
(Anonymous) 2016-02-11 12:48 am (UTC)(link)Re: Mosirinde's Covenant
(Anonymous) 2016-02-11 12:55 am (UTC)(link)Re: Mosirinde's Covenant