I wouldn't necessarily go that far. 98% didn't support it or, again, didn't care enough to sign it. That's an overwhelming majority. And a good portion of those that did sign were listed as (Retired) or not even the head chief.
Just perusing the letter, I saw a lot of NY, CT, MA and CA signatories. Which is kind of to be expected given the politics of the States themselves and the politicians they elect there that in turn appoint the police chiefs.
Just as a reminder though, there are some that buck the system. Remember the Colorado Sheriffs that flatly said they weren't going to support or enforce the idiotic measures put in place a few years ago? 54 of the 64 Sheriffs in Colorado brought a suit against the Governor (though later thrown out) and represent a lot of area in that State. So they lost the suit. It's just as easy to tell individual deputies "don't go looking for it" even in the aftermath of that lawsuit.
In reference to Ras asking about the gun confiscation thing, I doubt there as many cops out there that are as anti-gun as he might like to believe. Chiefs and whatnot maybe (at least 1.5% lol) but the rank and file probably just don't care that much about it or side on the 2A for the most part. I'm not going to slap a percentage on that, but I'd think there would be widespread revolts if a Chief or Sheriff ever ordered them door to door on confiscation. There are those overzealous lots that get all giddy on confiscating that "assault rifle and hundreds of rounds of ammo!" that gets widely reported. But I don't think they are the majority.