A vegan diet is definetely not healthy by definition but for many specific products I would still argue that the vegan version is indeed healthier. And especially for junk food. If you take sausages for example, then both versions - let’s say pork and soy/wheat - contain plenty of fat and salt. But meat products have…

1. Worse kinds of fat (meat contains mostly saturated fat while plants often have a better balance of saturated and unsaturated fats)
2. The risk to contain nitrosamines which are definitely known to have a negative health impact.

Animal based food on the other hand has better protein quality. E.g. eggs or meat contain a way better composition of amino acids than soy, wheat, peas etc. alone. This negative aspect of a vegan diet can be compensated by combining various different sources of protein (e.g. a mix potatoes, spinach, beans, peas and soy in one meal rather than just soy).

This obviously can be done but it requires some knowledge, practice and more time if you want to prepare everything yourself. Using pre-processed food can make things way easier, more convenient and not necessarily unhealthy. If you can buy a healthy vegan something that tastes good, has similar nutrients like an egg and doesn’t take hours to prepare that’ll make the transition for people a lot easier. If you give them complex recipes and long tables instead of what should be combined with what, it will scare off many folks.

Therefore, if we want more people to go vegan in a we shouldn’t object pre-processed food in general. We should rather praise manufacturers that manage to produce healthy substitutes. Without adding loads of sugar, cheap fats etc. There’s nothing wrong with large-scale food processing in general.