It would help to see the text of the interview, or at least a clear statement of the claim. Your phrasing "He claims that would have caused hundreds thousands of deaths (not clear, US or worldwide)." is ambiguous as to what "it" refers to, and you phrased it as a counterfactual (something that didn't happen), but if "it" refers to "The government taking ten years to institute laws requiring seat belts", that (presumably) is something that definitely happened.
Taking the claim to be "The government taking ten years to institute laws requiring seat belts caused hundreds of thousands of deaths", and taking "hundreds" to mean "at least 200", that is almost certainly false for the US, but plausible for worldwide.
Seat belts are the single most effective safety technology in the history of the automobile. A NHTSA study of lives saved by vehicle technologies found that, between 1960 and 2012, seat belts saved more lives—329,715, to be exact
If that rate is constant, that's ~66k/decade, which is off by a factor of three. Add in the fact that they are probably biased to overestimating the number of lives saved, and the fact that not all of those are due to laws (many people would have used seat belts if there weren't a law), and it's likely lower. Theoretically, there could have been a much higher number for the sixties, but it's highly unlikely.
Looking at this graph, there has been a general trend since 1967 for car deaths to go down. From 1967 to 1977, deaths went from 52,924 to 49,510, but the cumulative decrease is once again ~30k. Theoretically, it could be that the death toll would have risen without seat belts, and thus this underestimates how much the death toll would have decreased in the decade before 1968, but it's more likely that it overestimates the amount (there are likely other factors that cause the decrease).
Both of these are only rough indicators, but as they would have to be off by a factor of three to six for the claim to be true, this is strong evidence against it.
Worldwide, on the other hand, the US was about 1/17 of the world population in 1967. So the claim isn't outside the realm of possibility, although I don't know what how traffic deaths compared to the US, or when other countries implemented seat belt laws.
PS At least for the US, I'm pretty sure that "security" is not standard as part of the term for seat belts.