If I were to pre-purchase Humankind in Nexus.gg would I get to enter the OpenDev? Purchase gives a Steamkey.

Pay attention to the second point of section 1. So if you buy from somewhere else and it gives you a Steam key, you will need to redeem the key on Steam first and then follow the rest of the instructions

"So how can you play the Humankind Lucy OpenDev? Just follow these simple steps:

1. Make sure that HUMANKIND™ is in your Steam library.
   1. If you have pre-purchased the Digital Deluxe Edition on Steam, it should be there already
   2. If you have purchased a Steam key from another retailer, you must Redeem the key on Steam first before proceeding to the next step"