The fictional biographies for Riker and La Forge indicate that they both served together on the USS Hood (with Riker as First Officer and La Forge as a lieutenant), albeit the dialogue indicates that they've never actually met.
Note also that the original script for TNG: Encounter at Farpoint shows La Forge, Riker and various other junior officers on the planet waiting to come aboard the Enterprise:
RIKER : And you, Mister La Forge. Captain Dreyer praised your performance on the Hood. Why did you request transfer to the Enterprise? - TNG: Encounter at Farpoint
The episode's official novelisation mentions (and covers this) in a little more detail
The VISOR was actually more than just a replacement for his eyes. It
allowed him to see telescopically and microscopically, as well as view
the entire spectrum of light from X-ray to infrared. La Forge had also
been serving on the Hood as conn officer, but his duty shifts had not
often coincided with Riker's, and the older man knew him primarily by
As to why Picard said what he said, he may have simply been referring to the fact that Riker had known La Forge prior to joining the Enterprise which none of the other crew had.