You can't polish a turd. There would've been no point in modifying the engine a bunch if you hadn't given them a useful base to work with.
Of course you can. Not really sure why this still is tossed about. You just get a shiny turd with a lot less stink. I've made a career of taking people's turds and turning them into things they can now monetize (think old shitty video tape content prepped to be monetized with streaming).
You didn't polish a turd, you found something that could be valuable to someone and found a way to make it into a polished product, which is great.
But "You can't polish a turd" implies it's actually a turd and there's nothing valuable to be found or the necessary quality thresholds can't be met.
You absolutely can. Mythbusters proved it.
phoboslab was downplaying their own efforts by saying that the Cross Code team customised the Impact engine a bunch. My point was that no amount of customisation can turn a bad engine into a good one (you can't polish a turd), so phoboslab definitely deserves credit for building the base engine for Cross Code.
At a risk of being off-topic and not contributing much to this particular conversation (as I doubt it's relevant to the point you're making), I'd like to note that I often actually find it preferable to "polish a turd" than to start from scratch. It's just much easier for my mind to start putting something that already exists into shape than to stop staring at a blank screen, and in turn it can save me a lot of time even if what I end up with is basically a ship of Theseus. Something something ADHD, I guess.
However, I'm perfectly aware this approach makes little sense anywhere you have to fight to justify the need to get rid of stuff that already "works" to your higher-ups ;)