I there is a misunderstanding here...
Thanos wasn't killing the Asgardians as part of his culling process or at least that is not his primary reason.
He was looking for the Tesseract which he knows the Asgardians have.
Certainly Ebony Maw does the speech about being saved by Thanos
Hear me, and rejoice. You have had the privilege of being saved by the Great Titan. You may think this is suffering. No...It is salvation. Universal scales tip toward balance because of your sacrifice. Smile...For even in death, you have become Children of Thanos.
BUT....Thanos reveals his intentions..
I know what it's like to lose. To feel so desperately that you're right... yet to fail, nonetheless. [grabs Thor by the head.] It's frightening. Turns the legs to jelly. I ask you, to what end? Dread it. Run from it. Destiny arrives all the same. And now, it's here. Or should I say...I AM.
The Tesseract, or your brother's head. I assume you have a preference.
So clearly Thanos knows that the Tesseract is on board, he's not even the slightest bit fooled or convinced when Thor tells him that it was destroyed on Asgard.