Hard, dumb, blanket rules are hard and dumb. You can talk about your previous employers, if you do it right. More on that in a minute.
Plainly saying they went bankrupt is probably enough detail for most interviewers. It's blunt and direct, yet vague enough to be somewhat diplomatic. I certainly wouldn't feel ashamed about it. At the company level, bankruptcy of a startup is actually the expected outcome, statistically speaking. Shouldn't raise any eyebrows. And at the personal level, you even tried to confront them about the burn rate, so you shouldn't have any personal shame about it.
If somebody really presses you hard for why you thought the company failed, just tell them what you told us, and qualify it as your own opinion: "I think they probably burned too much money scaling the team before owning the product market fit." Sounds like a reasonably intelligent analysis of what happened (though personally I might question the use of the word "owning" just stylistically speaking... maybe "finding" instead?).
You can make yourself look good here, or bad. If you're capable of formulating a blame-free, diplomatic, equanimous (is that a word? I mean even-tempered/calm/composed/having equanimity), intelligent, big-picture analysis of why things went wrong, that makes you look good. If all you can manage are self-centered, immature, emotional hissy-fits about perceived slights that were of no particular consequence to anyone but you (small-picture), that makes you look bad. Getting into the character flaws, dishonesty and/or detailed mistakes of your management, is territory you want to avoid in an interview because it tends to torpedo your chances instantly. That's the reason the conventional wisdom is never to "badmouth" previous employers, PERIOD. I would obviously refine that somewhat, as detailed above. Analysis OK, gossipy shit-talk bad.
The person you're talking to is evaluating you, not your previous employer. If they see before them someone with complaints, they'll assume you're just a complainer, not someone with a legitimate complaint. Likewise if they see before them someone from a bankrupt company who has a calm analytical big-picture outlook, is a bit business-savvy, and has the company's best interests at heart, they'll see it as a great opportunity for them.