In that repository the case when they received the error the package did not have any tests or R code, https://github.com/hirenp-waferwire/RLint/runs/1065893143#step:8:9, https://github.com/hirenp-waferwire/RLint/tree/2657a20e1f27214aed6b5ee6b746ac9db1c98e18
The issue with Delaporte seems to be that the package uses FORTRAN code, which requires gfortran, but the default C compiler on macOS is clang. The gcov formats between gcc and clang are different, so I think you would need to compile both the FORTRAN and C code with the same toolchain (and use the gcc gcov binary) to get the expected results. I can reproduce the same issue outside of GitHub Actions on macOS and see a more informative error with covr::package_coverage(quiet = FALSE).
Probably the easiest solution is for you to run the coverage action on a linux runner rather than a macOS one, which has gcc as the default toolchain. We default to using macOS because of CRAN supplied package binaries, but it is clearly causing friction in this case.
I opened a pull request with the necessary change at https://github.com/aadler/Delaporte/pull/1