See we have to make so many logical leaps about them in this theory and all theories really.
If they can push 700gs, defy inertia and the laws of physics.. I just ASSUME they also solve the issue of radiation and intergalactic space travel. We have to do assume they can do all these things. Maybe they can travel interdimensionally and thus can avoid the physics of this dimension? I like to use my imagination, fun thought experiments
I do too

but the pieces have to fit entirely.
Yeah, I guess by avoiding travel through the physical plane/using a wormhole might prove fruitful, but I'm shaky on the details of how exactly they would get rid of their entire mass in order to access said wormhole, haha :D
The thing about the Haunebu is that it apparently creates its own gravitational field due to the magnet technology it is based on. This would allow to break certain "laws of physics", because it's just inside the confines of physics

just further outside of our planet's gravitational pull, yeah.
There's a lot of shady details around this man, so it's hard to figure out the truth. There are many old German books that are illegal to possess, which speak of similar experiments, or the Haunebu itself, written by ex-SS members, -edit- , just to name the 2 most famous ones. It's really weird, because there's so so much material on this.
edit: sorry I edited the names out, I'm a bit scared that mentioning them publically might be illegal in my country. I will PM you the names