They probably know what they're doing since they sell plants that can be pretty vigorous. So, I looked them up to see what's likely in the potting mix of the plants they sell.
Basically, they say about what I'd expect, with a few important extras added in. It looks like decent advice.
I might note that avoiding garden soil in containers (including for seed-starting), as they said, is a popular exhortation, but it's not necessarily bad (and I've never seen a potting mix that includes soil). Whether a person might want to use garden soil really depends on what they're doing, what results they want, how they're doing it, why they're doing it, and where they're doing it. But yeah, keep regular garden soil outdoors, please! It can spread hard-to-erradicate pests and diseases indoors.
In my Strong Camel 6'x5'x3' unheated greenhouse, I like to use regular garden soil for starting muskmelons, watermelons, and squash in 8oz foam cups. They do need to be watered on time to ensure that the soil doesn't shrink, stifle the roots, and compact too much, however, and it's best to overseed if you use it, it seems (more plants mean more roots breaking up the soil, which means better aeration, and better germination should it be difficult). I'm sure BonniePlants' ideal mix would be at least as effective, if not a lot better, however, without requiring as many seeds (but regular garden soil is pretty much free, and it does work better than many unideal soil-free mixes in this context; plus, you generally don't need to add extra fertilizer or minerals to regular soil after the plant gets a little bigger).
If I were to use any regular garden soil in smaller containers, in my unheated greenhouse, I wouldn't bring those indoors, and I would only mix a small amount of it with my seed-starting mix (like 1 to 10% or less), and I would mostly only use it for the purpose of adding extra nutrients/bacteria (and to reduce fertilization needs); also, I'd use it to help acclimatize the plants for transplanting (into regular garden soil) faster. I haven't historically used regular garden soil in my seed starting mixes for cells, though. So, if I try it, I might have other things to say about it.