'Without digging deeper' isn't really what matters here. A better question would be 'after some very cursory further digging, are we worried?' The first thing I ask myself when I see something that looks as wrong as this does is "Is it possible for this piece of code to be completely broken, and for no one to have noticed yet?"

This is really a question about two processes - the one where people look at, read, edit and discuss the code, and the extent to which this is discoverable by them. And the one where people run the code and expect certain things to work and would notice if they didn't work, or didn't work correctly.

This fails both of those checks. This file has been edited 8 times in the last year, by 5 different people. It's not so long that several of them wouldn't have read through the whole thing. This bug, if it was a bug, is unlikely to have persisted for 6 years without being noticed. It's not obfuscated or deeply technical in any way.

Secondly, this is code which is fairly critical to millions of systems. Ext4s is in literally hundreds of millions of devices. It's used in data centers in contexts where people are likely to follow up on any unusual behavior by their filesystem. The code in question is likely to be very well exercised in tests. It's likely to be extremely well exercised by people doing research stress testing/comparison testing of filesystem code. It is undoubtedly very well exercised by real-world deployments.

The bar to assume that you know something the person who wrote the code didn't, as opposed to the other way around, should be very high here. Even taking all the benefit of the doubt (which you should always do, never assume that stuff 'probably works' without evidence) it's not hard to dismiss this.