I don't think it's fair to flat-out say that Williams lowers their standards. However, there is evidence both for and against this assertion.
For:
With the exception of Tufts (which is a significantly larger school), the Williams team is considerably larger than the other NESCAC squad. Although you could argue that their size is due to not having cuts, the fact that they consistently bring in more high-caliber athletes than any other NESCAC school could strongly suggest that they have easier standards. Perhaps every 9:10 guy is also a valedictorian and has perfect SATs, but I would say that statistics and reason suggests otherwise.
Against:
1) Williams is a desirable school. It's the #1 LAC on USNews and #1 on Forbes. Their program history is unprecedented in the NESCAC. It is very likely that they are able to recruit top talent, both academically and athletically. It's also safe to assume that there are a handful of 9:10 kids that favor Williams over Stanford, Princeton, Yale, Harvard, etc. because of it's reputation and rankings.
2) Their coaching staff recruits relentlessly. Although this is somewhat anecdotal, I know that the Williams coaches are more involved in the process than say, Amherst or Wesleyan. That's not a knock on any of the above listed schools, but having been through the recruiting process, if a kid is felt like they are wanted, they are more likely to consider a school. I think it's safe to assume that with the amount of recruiting Williams does, that's how they get their extra 2-3 studs.
Although people tend to give MIT a pass because "their MIT, everyone wants to go there," if you scrutinize Williams' standards, you should also question MIT. If you think the 9:10 guys going to MIT meet MIT's standards, than I would think it's the same for Williams.