The easiest way to put this from a Steelers PoV is there's been a lot of talk about Pickens drama to production ratio over the last 18 months or so, and trading Pickens for that little is the team admitting the drama was as bad as it appeared and they were sick of it.
And so they went out and got another WR1 guy so they could afford to trade Pickens, put as good a face on it as possible to avoid knocking his value, and traded him the moment they got an offer they liked, even if the timing is awkward af. Russini's report is the Cowboys offered a 4th during the draft, a 3rd after.
Given the league's history of getting work out of WRs that Tomlin has given up, it'll be interesting to see what happens next.
As for what it means to the Steelers... I feel pretty sure their aim is to stay competitive while building for tomorrow. It's a rebuild, but not a strip it down rebuild. Trading Pickens doesn't do much for it talent to asset wise, but it doesn't really hurt things either, and if they were that sick of him, go for it. I've wanted him gone since he didn't bother running his route in the final do or die drive vs Dallas and he's done nothing to persuade me otherwise.