The definition of impedance as "the ratio of pressure to flow" means the oscillating pressure and oscillating of the sound wave.
Don't confuse the oscillating air velocity which makes up the sound vibrations in the instrument with the (approximately) steady flow blown through the instrument by the player. The player's air flow excites the resonance because of the action of the reed in a clarinet or the shape of the mouthpiece of a flute, but it isn't the same as the sound. For example blowing into the end of an open cylindrical tube doesn't produce a musical note, but blowing the same amount of air across the end of the tube (in a similar manner to playing a flute) will produce a note by exciting a resonance in the air within the tube.
The article doesn't say how the impedances are measured, and you can get different curves by choosing different positions.
But assuming they were measured where the energy is being input from the player, that point is effectively the open end of a tube for the flute, but the closed end (because of the reed) for a clarinet.
So for the flute resonance you have low (nominally zero) pressure difference from atmospheric but high air velocity, but for the clarinet you have high pressure and low (nominally zero) velocity.