This was a great question and one that was asked again just a few days later in our Kansas City Area gardening group, as well. The quick answer is everything you've heard about sunflowers is true!
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Resources:
Allelopathic Plants. 7. Sunflower (https://www.researchgate.net/profile/A-Azania/publication/287761119_Allelopathic_Plants_7_Sunflower_Helianthus_annuus_L/links/56e8a0ef08ae9bcb3e1cd8d3/Allelopathic-Plants-7-Sunflower-Helianthus-annuus-L.pdf)
Sunflower Power! (landhealthinstitute.org) (https://www.landhealthinstitute.org/single-post/sunflower-power)
Potential of Sunflower (Helianthus annuus L.) for Phytoremediation of Soils Contaminated with Heavy Metals (https://balwois.com/wp-content/uploads/old_proc/945.pdf)
Behaviour of Helianthus annuus L.: an ethogram for sunflower, and the effect of potential competitors on soil nutrient patch use (https://era.library.ualberta.ca/items/fe7a0965-08db-4263-83fd-59191b99bedd)