Tanya's decision has three strong points she greatly considered;
First of all, Mo Xie came from the outside world, based on stories, it's a treacherous place, humanity scheming against each other, war for territories and such, and as Mo Xie proclaimed himself as a Patriarch of a family, and to be a family in that world, one needs either power or a scheming mind.
Second, unlike Tanya who's not well versed in trickery and mostly has a mind of ambushing tactics and slowly dwindling enemies, she definitely has no experience leading many people at once, she placed all of her fate in Mo Xie, to lead the people to war with unknown experiences.
Whatever he had done outside is of no consequences, but one thing is for sure, he may lack strength, but even her father had some sort of faith trying to rely on Mo Xie's talent, well, most of it is getting out of this space.
