If that deal was on the table, they should have taken it. I'd rather have cap relief than Mo Williams.
Having read Simmons' tweet, though, I'd like more details. Is that a sourced story, or just random opinion / speculation? Some of the things that Simmons has "reported" about the Celtics just haven't been true, and I don't necessarily trust his connections.
Understanding that it might be influenced a bit my last nights performance, I actually think Mo Williams was a nice pickup for them, and was more valuable than the extra cap space this summer.
There are not a whole lot of good PGs on the market, and Williams is a quality starting PG who fits in well with their team (they needed shooting), and can help bridge the gap until Bledsoe is ready to take over.
I hated Williams on the Cavs, but he has always been a productive PG, and when he wasn't around Lebron James, he always seemed like a decent team guy.
Given where the Clippers stand at the moment, I think having a guy like Williams there will help some of the young guys develop a little better, and will also help them win some games, which is crucial to the development.
Particularly given that they already have their talent core with Blake and Gordon set, and some other nice young (and veteran) players around them, I am not sure they would have put the extra cap space into much better use than Mo Williams gives them, particularly since his contract only has 2 years after this.