That ticked me off sufficiently that I wrote them the following response:
Obviously, you did not research the Tanka Society of America, Tanka Canada, The Japanese Tanka Poets Club, or the Anglo-Japanese Tanka Society which are national organizations dedicated to tanka in their respective countries. In each case, their journals publish tanka in English in a variety of five line formats. Gusts, the journal of Tanka Canada's editorial guidelines explicitly ask for tanka of 19-26 syllables -- the editor-in-chief, Kozue Uzawa, is not only a university professor, tanka poet, and native Japanese speaker, but, along with co-editor Amelia Fielden, the recipient of the Donald Keene Award for Translation of Japanese Literature for their joint project, Ferris Wheel : 101 Contemporary Japanese Tanka.
Amelia Fielden is one of the most respected contemporary translators, tanka poets, and editors working in the West. She's translated a dozen or so modern Japanese tanka books. She advocates as 'short-long-short-long-long' pattern; she is a co-author with Denis M. Garrison and Robert Wilson of an article 'A Definition of the Ideal Form of Traditional Tanka Written in English.'
You're obviously not familiar with the works of Michael McClintock, an eminence gris in the fields of both tanka and haiku, who happens to be the President of the Tanka Society of America, as well as a contributing editor to Modern English Tanka (the journal), and an admirable poet and editor of many books of tanka. Since I mention it, Modern English Tanka is the most widely read and most prestigious journal of tanka in English. You will find very few sanjuichi tanka with its pages.
You have not obviously not read Prune Juice : A Journal of Kyoka and Senryu (humorous tanka and haiku), edited by Alexis Rotella, nor any of the books she's published over the last thirty years; she does not write or publish sanjuichi form tanka, either. She's co-editor with Denis Garrison of the Ash Moon Anthology, one of the best tanka anthologies of recent years.
You haven't read American Tanka which is the grand old dame of tanka in English--and which is one of the most conservative of the tanka journals (or was, until Michael McClintock became the new editor). You haven't read Eucalypt, the foremost tanka journal in Australia, nor Kokako (NZ), Time Haiku (UK), Presence (UK), Blithe Spirit (UK), nor visited the 3 Lights Gallery online, nor read Atlas Poetica : A Journal of Poetry of Place in Modern English Tanka.
You clearly haven't read any of the canon, such as Take Five : Best Contemporary Tanka, Fire Pearls : Short Masterpieces of the Human Heart, In the Ship's Wake, Countless Leaves, Full Moon Tide, Wind Five Folded, Streetlights, Landfall, Ash Moon, Sounds from the Unknown, or apparently any other well known, well-respected, and authoritative anthology.
You haven't visited reputable sites like TankaCentral.com, TankaOnline, TankaSocietyofAmerica, Tanka Canada's website Anglo-Japanese Tanka Society's website, nor read any of the reputable journals online.
You haven't read the winners of the International Tanka Contests, the Haiku Poets of California's Tanka Contests, the Saiygo Awards, the Tanka Splendor Awards, the Kokako Tanka Contests, the Yellow Moon Tanka Contests, the Moonset Tanka Contests, or the various other contests.
In short, your are displaying an ignorance that is astounding in its arrogance.
If you refuse to educate yourself on what tanka in English is and has been for more than 100 years, what all the national organizations dedicated to it teach, and what the major journals, websites, anthologies, and other works present as tanka, then you have absolutely no business hosting a tanka contest.