Levi counted inside Abraham’s action
The writer adds a careful-sounding qualifier (“so to say”), signaling a figure of speech or an indirect way of speaking. Even though Levi is known as one who receives tithes in Israel, the writer says Levi “paid tithes” through Abraham. The explanation is genealogical: Levi was not yet born, but was still within his ancestor (“in the loins of his father”) when Abraham met Melchizedek; therefore Abraham’s tithe is treated as including Levi’s participation.
Unit 3 (vv. 8–10 together): The logic step being made
Taken together, the passage moves from a contrast about mortality and scriptural testimony (v. 8) to a claim about representative action across generations (vv. 9–10). The writer uses that representative link to treat the later tithe-receiver (Levi) as, in a sense, a tithe-giver in the earlier story.