Tool author here. Sure, out of all bait in this thread I will bite this one. Curiously, it seems like Konstantinos has opened this ticket, i.e. https://github.com/iczelia/bzip3/issues/177, and open-sourced his algorithm here -- https://codeberg.org/kagiannis/gdcc-2021. As the allegation is very serious, I will also copy the edited version of this response to my website.

I have not seen his tool before, or even was aware of it. However, once you take a closer look, you notice that these two tools could not be more dissimilar. Agiannis' compressor uses a compact context representation to group bytes, followed by RLE and prefix coding. Bzip3 combines repetition removal (here via a run-length code -- prior to the BWT -- and LZP of Charles Bloom), a full Burrows–Wheeler transform, and a richer (thus slower) statistical arithmetic coder. The use of RLE for post-coding the BWT output dates as far back to Julian Seward, perhaps even further. The RLE and LZP are applied before the Burrows-Wheeler transform (as opposed to the implementation in `text', which makes a big difference). Bzip3 uses a proper SAIS library for the forward and backward transforms. Konstantinos' entropy coder seems to use FPC (bytewise prefix codes over adaptively selected subblocks), bzip3 uses an idea similar to this of bcm, which itself descends from Mahoney and ancient work of JS Vitter on arihtmetic coding, where a bitwise arihtmetic coder is input mixed probability estimates from exponential-moving averages with probability refinement.

You are welcome to conduct your own analysis, but this is the gist of it -- perhaps Konstantinos has convinced himself that he had invented run-length coding?