Ellis's own 1845 book, The Alphabet of Nature, states he designed the English Phonotypic Alphabet's letterforms "conjointly" with Pitman

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The Alphabet of Nature — Alexander John Ellis's own analytical treatise on phonetic notation, serialized in Isaac Pitman's Phonotypic Journal from June 1844 to June 1845 and then issued as a standalone volume co-published by Bagster and Sons (London) and Pitman's Phonographic Institution (Bath) in 1845 — states directly, of the printed Phonotypic Alphabet's letter designs, that the forms "have been the subjects of many experiments for more than a year and a half, made chiefly by Mr. I. Pitman, and the author of this essay, conjointly" (a corrected reading of a legible but noisily-OCR'd scan; the exact OCR string is preserved above as source_quote).

This is Ellis's own first-person account of his own collaborative work, published within months of the alphabet's completion. It upgrades claim-pitman-and-ellis-co-created-english-phonotypic-alphabet — until now sourced to Wikipedia alone (Tier 4) — with a Tier 1 primary: the co-creator naming the collaboration, in his own words, in his own book.