Hongren's "Treatise on the Supreme Vehicle" (T48n2011) doesn't mention the Lankavatara Sutra at all, Daoxin's "The Fundamental Expedient Teachings for Calming the Mind Which Attains Enlightenment" (in T85n2837, i.e. Records of the Teachers and Students of the Lanka) only mentions the sutra once but doesn't quote from it (unlike from other scriptures), the complete Record of Lanka teachers, including preface, quotes the sutra 5x (compared to: Avatamsaka 8x, Lotus 5x, Vimalakirti 4x, Nirvana 3x, Dharmapada 2x) and refers to it 9x. In the Bodhidharma Anthology's translation part the Lankavatara Sutra is quoted only once. McRae explains it this way,
"Note, for example, the shift from Lankavatara Sutra to Diamond Sutra implied in the account (i.e., in the cancellation of the painting commission and Hongren's teaching to Huineng), which parallels the two texts' changes in popularity over the course of the eighth century. The position of the Lankavatara within Chan was always ambiguous, since the text was more revered in the abstract than actually studied. However, it was generally associated with ''Northern school" teachers. Shenhui was one of the first monks of his day, but by no means the only one, to favor the Diamond, which was becoming more widely popular throughout the Chinese tradition at the time. Hence, in the Platform Sutra the two texts roughly symbolize the Northern and Southern schools."
(Seeing Through Zen, p. 62)
Also, if you take the Records of Lanka Teachers, it lists Gunabhadra as the first and Bodhidharma as the second patriarch and Shenxiu is the sixth patriarch while Huineng is just a mediocre student of Hongren (among those who are "fit to be people's teachers, but will only be local figures"). It's just another thing that Daoxuan's story of Huike in his collection of biographies of eminent monks (T50n2060), where he is a Lankavatara teacher just as his students, has little connection to the later Zen tradition. Thus it is understood that it was Faru and other disciples in the "northern school" who made the connection between Bodhidharma and Huineng to Daoxin and Hongren. It also explains the lack of any reliable information on Sengcan.
"There is no such thing as the real mind. Ridding yourself of delusion: that's the real mind."
(Sheng-yen: Getting the Buddha Mind, p 73)
“Don’t rashly seek the true Buddha;
True Buddha can’t be found.
Does marvelous nature and spirit
Need tempering or refinement?
Mind is this mind carefree;
This face, the face at birth."
(Nanyue Mingzan: Enjoying the Way, tr. Jeff Shore; T51n2076, p461b24-26)