Salamandyr
Adventurer
that's clearly not the case:
Basic Rules, page 46 (Improvised Weapons)
"At the DM's Option, a character proficient with a weapon can use a similar object as if it were that weapon and use his or her proficiency bonus." Unless you're proficient with Chair or Lamp, you might want this feat.
Also, it takes your Unarmed Strike from 1 damage to 1d4 damage.
You are correct; I mis-remembered. Unarmed strikes are simple weapons, but improvised weapons are their own separate category and not on that table.
Okay, that makes this feat a little more interesting to me. I did note the bit about unarmed strike damage going up. I just don't think it's enough to get excited about.