Interesting. I looked up the UCMJ articles for manslaughter and murder, and the language is actually quite plain; reads to me like you are obviously correct that this case would clearly be involuntary manslaughter.

In case anyone else is interested:

UCMJ Article 119 (Manslaughter): https://www.law.cornell.edu/uscode/text/10/919

UCML Article 118 (Murder): https://www.law.cornell.edu/uscode/text/10/918