Yes, it is possible to use log4j-core as the backend for SLF4J. The vast majority of the time, especially in small, bespoke services, the backend is logback.
I'm reminded of something Al Viro said long ago about the Linux kernel[1]
"Yes. So's sysfs, so's udev, so's hal, so's any number of revolting
strings of intertwined copulating tapeworms hanging off the kernel's arse."