Ferrari's overall pace is close enough to RBR for Alonso to make the difference. Today was by far Ferrari's best showing of the season, took Rosberg quite a while to catch Alonso - although the pass was pretty easy. Also finishing just 23 seconds behind Hamilton indicated progress was being made.
Well, watching the F1 timing app it was quite easy to see what happened to Rosberg a few times...
He was gaining about half a second to a second a lap and then when he got to ~2 seconds he locked up just before pitting which lost him a fair bit of time so once they had both done their pits he was back to something like 8 seconds, he then chased again and was going half a second to a second faster but Alonso kept finding the backmarkers on the straights and Rosberg collected a few backmarkers in awkward locations.
He also used more fuel catching back up after the lap 1 incident and then on top of that he had no telemetry so was having to give feedback a lot (more than you hear on the radio) and wasn't going flat out because they needed to protect the car as without telemetry they couldn't tell anything about the car, just before he overtook Alonso he was told over the radio to go full power to overtake and that's what he did and then breezed past and then sat in cruise.
Ferrari's pace was impressive today but anyone saying Raikkonen is rubbish or not able to challenge Alonso... Kimi's just taking longer to adjust to the car and the break by wire (he locks up a lot because of it), he's still a classy driver and will up his game sooner or later.
I'd say Red Bull are still ahead but the Ferrari Engine has better power output, the thing that helps Red Bull stay in contention is their downforce and low tyre wear.