Clarification about the 400 year period

"It will be 400 years before we reach you. By the time we do, you will have long surpassed us. You will destroy our fleet and then come back and destroy our world to make sure we never threaten you again."

So I have a question about this.

I suppose the San-ti's fleet won't be able to advance their tech since the travel take them 400 years and they will probably spent the time in dehydrated state.

But the ones remaining in their planet 4 light years away, won't they be able to advance their tech during the 400 years ?

Or they think another chaotic era will occur, reseting their world so by the time they reconstruct everything, humans would have the advantage.

Can someone clarify me about this ? Maybe I missed something.