What does CMS standard mean by "initial input to the message digest"
Consider the two cases.

without signedAttrs: first we hash the content, then we sign that hash. The first thing we do is hash the content.

with signedAttrs: first we hash the content, call it hash1; then we construct signedAttrs which includes hash1 in the message-digest attribute; then we hash signedAttrs, call it hash2; then we sign hash2. Only hash2 is used in the digital signature operation, but to compute hash2 we must previously have the value of signedAtttrs, and to have the value of signedAttrs we must previously have hash1, which is the hash of the content, so the first thing we do is hash the content.

Either way the first thing we do is hash the content which uses the content at input. Since this is the first thing we do, the initial input is the content. And in particular the value octets, not the tag and length, which is the actual point being specified in that paragraph.