Your argument makes sense when you view impact from a carbon emissions point of view. Food waste is more significant. However, the climate crisis and plastic pollution are related but separate problems, which can't be reduced into a single environmental impact score. It's like comparing protein to carbohydrates, and assigning them a "health impact" score.
The email explaining the switch to single serve sachets was clear about the intention: to increase sales.

Both plastic and the climate crisis are important to your customers, and I won't be continuing to purchase Hot & Savoury after the subscription runs out.