I hope it's obvious that the whole idea of an off peak ticketing option is to encourage non-commute transit usage. That is when excess capacity exists. Commuters would pay normal (non-discounted) commute-time rates, though often other incentive programmes exist, varying by locale.

From HN discussion and information elsewhere (including the NL rail site itself), so long as a trip begins off-peak, the off-peak rates apply:

Please note: the time you check in determines whether you will travel with a discount.

<https://www.ns.nl/en/season-tickets/dal-vrij> (see FAQs section)

Other systems may of course vary, but typically trip start / time of ticket purchase / time of ticket check determines the applicable rate.