Keep in mind that not only does Tara have no idea what Negan and his beloved barbed-wire bat have been doing in Alexandria since she left a week before -- she’s under the impression that Rick and her crew have wiped the Saviors out altogether, as she somewhat arrogantly informs the Oceanside women over dinner in a bid to be their friends. But these women know better. When they offer to return Tara to the bridge where she lost Heath, sending her packing with her word that she'll never speak of this place to anyone, they swiftly prove insincere, turning on her and attempting to gun her down. They know the threat Tara poses, even if Tara herself doesn't.
And the Oceanside women know that Negan is still very much alive, and that if the Alexandrians were stupid enough to fight them, the Alexandrians are either dead and gone or soon headed there. It's only by sheer luck -- as well as some help from a new young friend -- that Tara manages to escape certain death and make it back home in one piece. (As for Heath, his fate remains to be seen.) Of course, knowing that a fully armed community of badass women is out there and friendly would be useful and maybe even inspiring knowledge to the Alexandrians, who are currently feeling friendless and without hope for the future. Add the Kingdom and Carol, and you've got a potentially Negan-thwarting team. But for the time being, Tara remains mum on the subject: she's been sworn to secrecy, and she's keeping her word.