Zayd and a lion both exist and are corporeal, yet existence or corporeality is not the intended aspect in “Zayd is as brave as a lion.” The comparison foregrounds courage as the particular quality to be communicated.
The source qualifies participation by «الذي قصد اشتراك الطرفين فيه»: it must be the respect in which the speaker intends the two terms to be shared. A merely accidental property does not automatically count.
A useful test is to ask which characteristic of the subject the speaker intends to clarify through the comparison. That characteristic, rather than every shared attribute, is the point of resemblance.