The article addresses this:

One might suspect that materials, distribution, or other book-specific costs had risen over the intervening decades. But how then to explain that the mass-market paperback’s older sibling — the hidebound hardcover — hasn’t seen an inflation nearly so dramatic? [...] In 1939, you could buy ten mass-market paperbacks for the cost of a single new hardback. In 2024, you could barely buy two. Which means that under the leadership of the Big Five and their equally hegemonic national retail partners, the flagship product favored by elite-facing bookstores in wealthy enclaves got cheaper, and the format with the proven mass potential got many times more expensive.

Hardcovers use more paper than paperbacks. They're usually larger and the cover contains cardboard, which is several layers of paper. Even if the risen cost of paper plays a role, the difference in inflation between the two formats should not have been this big.