(x-post of a talkchess post that I made, that is in Engine Origins, that I am currently unable to access. I am locked out of my talkchess account, so hopefully discussion can be redirected here.)
Before I begin this, I want to make clear that I will share sources and further information with experts if asked by experts directly. I do not intend to release large segments of Houdart's original code except to e.g. actual authorities, like talkchess moderators, very well known community members (e.g. former talkchess moderators), or noted SF contributors. I'm afraid, however, that if I share the code with SF contributors, there may be a threat of tainting the SF codebase or tainting that contributor in general, as happened when Richard Vida reverse engineered Houdini 1.5, revealed it to be based on RobboLito, and was forced to stop developing Critter. If there is anyone to whom these concerns do not apply, please share your contact information (or contact me on Discord. I can be found in the TCEC Discord channel) and I can provide the necessary evidence.
Recently, I came into possession of Houdini 6's source code (experts can ask how--I already reported how to Houdart in doing my due diligence...my background is in security work. I only realized it was a clone far later) and decided to look into the code.
First, I should note that Houdart has done a good deal of original work and should be commended for this. However, Houdart's work from Houdini 5 onward comes from building on top of Stockfish. Houdini 5 is a Stockfish fork, no more and no less.
Line 410 is of note; it includes "to compile stockfish"
This is from util_tools.cpp, if you were curious about the version:
#define PROGRAM_NAME "Houdini 6"
#define PROGRAM_NAME "Houdini 6.03"
Houdini's roots were initially (as in for the first version(s)) in the form of being a modified RobboLito as well, but that was well known, even if Houdart would deny it.
If you translate search.cpp to the Dutch (Houdart is from a Dutch-speaking region of Belgium) zoeken.cpp, you'll find Houdini's search.
Although I believe Houdart also took incorporated newer fishtest patches and changes, so that is something worth bearing in mind.
Houdini's case appears to be somewhat similar to Rybka's, though Rybka ended up with far more innovations (and far more valuable innovations at that) than Houdini did. Houdini has taken Stockfish and made a large number of large changes. It has its own testing system, for example, completely distinct from Fishtest's. It has a custom tuner and specialized evaluation. Nonetheless, it is still Stockfish, and it is far more Stockfish than Rybka was Fruit.
I would think that this is a significant GPL violation.