Is it? Study [1] claims "Mice fed an FS [fat and sugar] diet ad libitum (FSA) consumed the same amount of calories as mice fed within a 9 hr window of the dark phase (FST) [fat and sugar, time restricted] (Figure S2A, i), yet the FST [time restricted] mice gained less body weight"
and then "to investigate the therapeutic potential of TRF [Time Restricted Feeding], we tested whether TRF could reverse or arrest body weight gain [..] Within a few days the mice were habituated to the new feeding paradigm and continued to consume equivalent calories (Figure S2A, iv). The 13:12 FAT mice showed a modest drop in body weight"
[1] https://www.cell.com/cell-metabolism/fulltext/S1550-4131(14)...
From related work of https://www.salk.edu/scientist/satchidananda-panda/
I think "it has to be calories in/calories out because it's just common sense" is not nuanced enough for biological systems.
Although there are other benefits to IF, the weight loss is principally from calories in, calories out, because it’s much harder to cram an excess of calories into a small time window unless you purposefully eat beyond satiation.