The plant itself does not repel insects. The Pyrethrum flower has to be processed to be effective. The pesticide is in the flower, not the foliage and it must contact the insect in order to kill it. There are a few varieties used specifically for pesticide usage: Chrysanthemum cinerariifolium and Chrysanthemum coccineum. There are about a thousand varieties and cultivars of chrysanthemum. Using these specific strains is not something a home gardener can easily do.
Wikipedia is not a valid, fact based medium. Most pages may/can be valid, but it's not a place I go to for the final word. For people who don't know me or who are new, I was a professional grower for many years. I managed garden centers and greenhouse growing under certified organic conditions and I went through the three year process it takes of impeccable record keeping to qualify for Certified Organic for operations I ran. I do not know everything and I ask big dumb questions here on a daily basis about growing pot. With gardening in general, and organics, I know what I'm talking about. Not in a braggy way, but I do have some confidence in my ability in this area.
Gardeners have always used many differing methods to grow plants of all kinds. There is evidence for the nematode management using a few different varieties of Tagetes, but you will not find those varieties anywhere being grown for sale in nurseries. Generally, they are not all that fancy. You can probably get these varieties from Johnny's Seed dot com.
When choosing a pesticide you think is organic, make certain you know terms for the different active ingredient. In market items with a chemical called pyrethroid, which is not natural should never be used. Ever. Being certain about pesticides is very important if it is going into your lungs. Our lungs are an incredible delivery vehicle directly into our nervous system. When I say things about organics, my only motivation is to protect people from themselves. There are a lot of new growers and a lot of amazing marketeers out there in lala land. Just because a product calls itself "natural" does not mean anything. The only products you should trust to be organic are those which specify certified organic or OMRI. Everything else is full of crap. Period.
I do always revert back to the old adage that, if it works for you, by all means use the method. Share the findings, but where I get a little concerned is when people make definitive claims to something which simply does not always work. Maybe never work. I can assure you one thing, marigolds are spider mite magnets. Planted across the garden from the plants you are protecting can never hurt, but if your plants are in stress, have any nutrient deficiency or is wilting on a regular basis from poor watering practices, you will have spider mites no matter how many marigolds you plant!