It doesn't even hold up to logic. For Bush to "lie" about Iraqi WMD, it would be necessary for him to know that Iraq had no WMD stockpiles, but to still claim that they did. No one but Saddam really knew the true status of Iraqi WMDs prior to the war, and he could not be believed for many reasons.
The only reason they say "Bush lied" about anything is to try to equate him with their favorite perjurer, Bill Clinton. It fries them that BJC was impeached for 'just lying about sex' when it didn't hurt anyone! Thus their line, "Bush lied, people died" - to contrast him to BJ.
Don't bother saying that BJ too thought Saddam had the weapons - they don't listen to reason. 4-word slogans are as far as they go.