Indeed, but it's still a gamble and there are other areas of the squad that need strengthening. Bony has looked good and he may even do better than Lukaku, but Martinez has seen Lukaku and worked with him already.
You need to be prepared to gamble sometimes when it comes to signing players, but Martinez may think that if he can't get Lukaku, he'll take the 20 Million and spend it to strengthen elsewhere rather than risk blowing it all on one player he's unfamiliar with.
He clearly has faith in Kone or he wouldn't have signed him