I know some people are scratching their heads over him b/c of his low profile, but just keep in mind, out of all the guys they signed, Williams is probably the one that the staff has recruited the longest. Barnes offered him immediately after getting the job and learning about him through Des Oliver.
They watched him play a ton this summer, had him to camp where they obviously saw him up close and spent time with him. Just in short, this is a kid that they probably saw as much as anyone they signed with the possible exception of Fulkerson and pretty obviously liked what they saw.