It is quite possible that Linux is the bigger target so it gets more focus. Vulnerabilities there are generally considered more valuable and notable. It would be very difficult to use these numbers to get a meaningful "more secure" stance as there are tons of variables.

From a quick search, Linux kernel is ~40 million loc, freebsd ~9 million, openbsd ~3 million. Number of bugs compared to lines of code leaves FreeBSD looking worse than Linux.

Linux also has a ton of extra functionality so I think you’d also have to do some adjustment for “as a user would I be at risk?” versus “can I be a user because it supports my needs?” Some of that would be unfavorable for many users (e.g. a Linux user who is exposed due to a network protocol or file system they’ll never use) but that’s certainly not true of every feature.

If this is just counting the kernel, than Linux is probably a bigger target both i terms of current code size and the amount of churn in the codebase as things change over time. Some of the LPEs might (I've not checked) be in modules that are not commonly loaded, which mitigates their overall significance somewhat.

In the less likely even that this is counting what laymen would call Linux or BSD, i.e. both the kernel and common libraries & tools, then Linux definitely has a wider attack surface. Though some of that surface is shared as some userland parts are common to both.

As with your assessment, I'd agree that these flat numbers without looking for further context don't really give enough for a one-is-more-or-less-secure statement.