Ask Mr. Smarty Plants

First of all, Mr. Smarty Plants is part of the Lady Bird Johnson Wildflower Center whose mission is "to increase the sustainable use and conservation of native wildflowers, plants and landscapes."  Native, in this case, means native to North America.  Second of all, Succisa pratensis (Devil's bit scabious), is native to Europe (including UK) and NOT native to North America and not really in our purview.  It is sold as a garden plant in North America and the ones in your yard are someone's garden escapees.  I searched the internet using the terms "Succisa pratensis allelopathic" and I could find no evidence that the plant is allelopathic (i.e., kills or inhibits the growth of other plants growing near it).   Emorsgate Seeds in Great Britain concurs with your statement about the origin of the plant's common name.  They say:  "Devil's-bit scabious is so named because its roots end abruptly as though bitten off by the devil."