Interestingly however, there were dark angels working on Caliban with the watchers when the garrison was defecting, working to keep the garrison from gaining awareness that the Horus Heresy was going on.
And when the Dark Angels caught up to these guys centuries later, they branded them as Fallen all the same.
The Dark Angels themselves however, seem to have a particular zealousness in hunting down the fallen that makes them seem pretty heretical already (what with attacking imperial forces and all) without anyone having to hear what the fallen have to say about the dark angels.
It comes down to this: no loyalist chapter is "known" to have divided loyalties. Sure, a handful of marines might have been part of the Lodges at some point, a couple might have even turned to Chaos. But the remnant on Caliban was not insignificant. If it were common knowledge that big a part of the Dark Angels became traitors, the whole Chapter's reputation would suffer. That's the basis for the Hunt, anyways. As the centuries have gone on, and the Dark Angels have gotten even more paranoid, the lengths they're willing to go to, to keep the Fallen a secret from their own people even, has bordered on heretical. Or at the very least, when push comes to shove, the Dark Angels put the honor and sanctity of the Chapter's good name above most other things. Perhaps even the Emperor, if it came down to it.
It's also worth mentioning that, before and during the Heresy, the Lion did not engender a lot trust from people. That the Dark Angels could have been traitors in some capacity wouldn't just hurt the Dark Angel's name, it would cast aspersions on the Lion as well, confirming in the minds of some what they already believed about him, that his secrets concealed something dark. In the pure/impure fanaticism of the 41st Millennium, the widely known revelation would be enough to condemn the entire chapter in the eyes of some Inquisitors.
Why would the watchers in the dark keep Lion El Johnson's location secret, unless the dark angels cannot be trusted to not put a sword through his skull? Maybe they're just xenos dickheads.
In truth, the Watchers in the Dark have always struck me as a tease. A subversive curio. Back in the day when they were fleshing out Chapter histories, I think they needed something occult and mysterious to go along with the Dark Angels theme, and so they ended up with little robed dudes being part of some big secret they never tell you about. Other than, you know, the secret of their relationship to the Lion.
Come the HH novels, you learn that something slept underneath Caliban that was somehow related to the Watchers.
My theory these days is that the sickness at the heart of Caliban got into Lion El'Jonson during his upbringing there, buried deeply but there none the less. After the destruction of Caliban, the remainder of this thing inside Lion El'Jonson is all that's left. And so the Watchers in the Dark keep him on ice, keeping an eye on it just like they did when it infested Caliban.