I'd wager that if you're amassing forces to conduct an invasion and midway through you discovered your target has nukes pointed at you, the very least you'd do is rethink your strategy and put a pause on operations.
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...