That's when it became mandatory on all vehicles, not when the first vehicle to satisfy the standard became available on the market.
As another example, side impact airbags have been mandatory in the US since 2013, but the Volvo 850 had them in 1995 and they were present on something like half of new cars a decade before they were mandatory and >97% of new cars three years before the mandate.
As another example, side impact airbags have been mandatory in the US since 2013, but the Volvo 850 had them in 1995 and they were present on something like half of new cars a decade before they were mandatory and >97% of new cars three years before the mandate.