In discussing the use of אני in TL, it is necessary to distinguish between אני אומר used in contradiction, and שאני אומר used to introduce a reason for an opinion. The second of these is not relevant here. (It is found, e.g. in Kinnim 3.3; Demai 5.3; Teharot 3.7; Maaser Sheni T 4.12 (94); 5.1 (95); Gittin T 6.9 (329); Oholot T 16.1 (613); 16.10 (614); and many other places.) In it, the אני has no reference to any particular person. Nor does it in the similar expressions הריני אומר (Terumot T 8.17 (40)); מה אני מקיים (Sifra 26.3); אני יודע (Sifra 25.9, where the text reads איני); אדון אני (Sifra 22.27 (end)) etc. But even here, generalization is misleading, for these expressions are sometimes used to introduce contradictions and then the pronoun they contain is emphasized. For example: שאני אומר in Mikwaot T 6.3 (658) and again in 1.2 (652). See also Niddah T 1.6 (641) where the expression may be from the hand of the editor. Prof. Lieberman tells me that Solomon Adani, in his book Meleket Shelomoh, tractate Arakin, ch. 4, mishnah 2, lists the rabbinical uses of אומר אני. See also the article of Prof. Lieberman in Tarbiz 2.110.
Morton Smith, Tannaitic Parallels to the Gospels, Journal of Biblical Literature Monograph Series, vol. VI (Philadelphia: Society for Biblical Literature, 1951), 43, n. 98.