I understand what you are saying. "Full" during the very first backup means an exact copy of all folders/files in the entire backup set. "Full" during subsequent (incremental) backups means an exact copy of only new or changed files. (ignoring block-level for simplicity)

This all seems reasonable and I would not be worried or confused except that when I read the documentation for incremental at:
https://www.cloudberrylab.com/resources/blog/incremental-backup-guide/
Which states:
"Forever incremental backup decreases recovery reliability"
"It also becomes harder and takes longer to recover the whole dataset"
"periodical full backups are highly recommended to start the new sequence of incremental backups"
"An incremental backup approach has several drawbacks...Less data consistency and reliability"
"backup software analyzes the whole incremental backup chain. If one element in that chain is corrupt, your data is non-recoverable"
"The flaws of this backup type such as slower restore and dependence on the increments integrity may be addressed by performing the full backup periodically"

This documentation seems to say that incremental is NOT RELIABLE unless a "Full" backup (meaning a new exact copy of ALL folders/files in the entire backup set) is performed occasionally.

You said that "...file backups are incremental forever..."

https://www.cloudberrylab.com/resources/blog/incremental-backup-guide/
This documentation says. "Forever incremental backup decreases recovery reliability" and
"periodical full backups are highly recommended"

So, backups are incremental forever (reducing reliability) and need a periodic full backup. I can find no option to run a periodic full backup as recommended.

This thread suggests that full and incremental are the same thing. (The word full was just replaced by the word incremental but they always meant the same thing.) How does this explain the Cloudberry documentation that says incremental forever is not reliable and requires periodic full backups?