I think a more measured stance is that the crash WON'T be (significantly) deflationary.
If the crash is "naturally" deflationary, the central banks have now shown a commitment to sustaining inflationary policy to keep inflation at target.
But in the case of a "naturally" inflationary recession (stagflation), central banks will be hesitant to ramp up deflationary policy, because regaining employment levels will be a higher priority, and deflationary policy tends to hurt employment.
If the crash is "naturally" deflationary, the central banks have now shown a commitment to sustaining inflationary policy to keep inflation at target.
But in the case of a "naturally" inflationary recession (stagflation), central banks will be hesitant to ramp up deflationary policy, because regaining employment levels will be a higher priority, and deflationary policy tends to hurt employment.