Mulberry are unique in many ways. I grow viable seeds from rubra x alba and i grow many male mulberries. Sometimes male mulberries do produce fruit infrequently. They can gender change.
I think what you are saying is that illinois everbearing and most others will produce fruit in the absence of male mulberries which is true. Mulberries do need males to produce seed. That being said the fruit is seedless without a male.
Illinois everbearing are appealing because of the fruit size and taste. You might wonder where it originated and fedco discusses that.
" bare-root trees
Morus alba × M. rubra 30x20’ White County, IL, 1947.
Sweet flavorful purple-black 1" blackberry-like fruit is delicious fresh, in sauces, pies, fruit leather or smoothies. What fruit you don’t pick, the birds will. Great tree for viewing a huge variety of birds up close. Plant it next to the chicken pen where it will rain chicken feed into the yard for several weeks, or put down a sheet to collect the fruit as it falls. Dense round-topped tree is one of the last to leaf out in the spring and one of the first to set fruit. Ripens in mid-July and continues for several weeks into late summer or longer. Aaron Parker of Edgewood Nursery in Falmouth, ME, had fruit on his tree into October!
Fast growing, even in poor soils. Prefers moist well-drained soils but otherwise adaptable. Full sun to partial shade; withstands pollution, drought, wind and salt. Self-fruitful grafted cultivar, so only one plant required for fruit. Z4/5."
I have heard many people call it a triploid but others dont agree.
" Morus Alba x rubra) Selected in Illinois around 1947, Illinois Everbearing is a cross between the Chinese White Mulberry and the native (to eastern North America) Red Mulberry. It is hardy to -25°F (USDA zone 4) making it one of the best varieties for the Northwest. It’s berries are large, 1 1/2″ long, black, tasty, with few seeds, and ripen over a long period from late June to early fall. Illinois Everbearing is self-fertile and can bear 15-25 pounds of fruit at maturity.
Self-fruitful – can produce fruit on it’s own. Planting a male mulberry tree nearby will cause the fruit to have viable seed. No male tree nearby = seedless fruit."
There is much speculation but i suspect it is fertile when.male pollen exists that is a viable match that overlaps fruit set time.
How can we solve this mystery? We will do this by contacting our friend and genetic expert @Richard and put the triploid rumor to bed. @Richard it is my theory that of the many male rubrus x alba hybrid males i keep around which is seven that one will be pollen compatible with illinois everbearing. My theory is since it is everbearing, pollen could be stored in the freezer and used if overlapping naturally is not possible. How i know this is Richard has a thread on the study of mulberry genetics
Studies of Fruiting Mulberries
I have a bit of experience improving mulberries through wild crosses