I think he plead guilty because the judge denied him the chance to make his case heard in court.
That having been said Article 66 of the UCMJ calls for an automatic appeal in any case in which the approved sentence extends to death, confinement for one year or more, or dismissal, dishonorable or bad conduct discharge (BCD) of a service member.
So despite assertions by the so called sanity squad that LtCol Lakin has lost all rights to an appeal by pleading guilty, he gets one anyway.
Members do not waive their rights to appeal when entering an unconditional guilty plea, but they do waive their right to appeal some> issues. In fact, just like in a finding of guilt by a jury panel or MJ, the appeal will be automatic. But, (and I'm speaking generally) with such a plea, what is reviewable by the appellate court becomes limited, which is why such appeals can be expedited - what is reviewable in a plea of guilty relative to a finding of guilty is much more narrow.
It is likely that to have preserved for appeal Lind's ruling (a pre-trial ruling) with respect to the disallowing of the affirmative defense on the Art. 92 specifications, Lakin would have had to have pleaded "not-guilty". He didn't. As such, it's unlikely he can appeal that particular ruling. He can appeal other rulings made subsequent to his naked plea - like sentencing and the like.