That’s not quite correct. See upthread my comment about a random example LHR-CDG which is 20% discount on an Off Peak date but doesn’t have any discount on a Peak date. Even though both are in the promotional period and the T&Cs state both peak and offpeak flights are discounted.
So there must be some nuances

EDIT: And as per my previous post, the « Up to 40% » is not always 40% (can be less, e.g. DXB or CDG examples). But the « 45% off » on the handful of routes listed at the bottom of the offer page seems to be guaranteed.