I don't follow. What exactly leads you to believe that lobbing nukes at an invasion force's staging area and/or neighboring military bases "does not have a credible chance of hurting" the invader?
I hope you will believe me without proper citations that they just have a lot of troops.[1] And there is no reason to think they would need to bunch them up. How many smuggled in nukes are we imagining here?
1: this is the orbat of the Chinese rocket forces from 2020 https://www.aboyandhis.blog/post/mapping-the-people-s-libera...