Just one more tidbit, this appears to be the same person who made some very outlandish claims on Tetris 2 with zero evidence to back them up, including claiming that they got a 18:44 and simply "was not recording" in a category where the WR is 21:06 and their PB is 22:34, another claim that their mother beat another WR by nearly 42 minutes, and yet another claim that they had found an RNG manipulation trick without any evidence that it actually works (as opposed to just seeing patterns in randomness when they tested it). So I think this person has about as much credibility as someone claiming to be the Prince of Nigeria.

Looking over my splits and doing some back-of-the-napkin math, I'd estimate the first 10 rounds take around 20-25 seconds each with average RNG and no misdrops, and around 30-35 seconds with bad-but-not-terrible RNG. Then 11-20 take around 40-45 seconds with average RNG, but great RNG (80th percentile or so) can bring a round down to 30-35 seconds. Then 21-30, we're looking at about 1:00 with average RNG and 45s with great RNG. This is all including the bonus animations, and amortizing the cutscene time across the 5 rounds.

So assuming average RNG the whole time and taking the lower end of those estimates, that's 20 minutes flat. Of course, at this point you're already sort of grinding for consistently not-terrible RNG, since bad RNG can hurt your time more than good RNG helps. But since you just need one lucky run out of hundreds, throw in a couple good RNG levels and we're looking at a time around 19 minutes. So the 18:59 estimate sounds about right.

As for 1-80, in my max score runs, I found rounds 31-50 usually took around 1:30 with average RNG. Since this is a longer run, it'll take a lot of grinding just to get consistently not-terrible RNG, so we're looking at 20 minutes to get to round 31 then 75 minutes for the remaining 50 rounds, so that's 1:35:00.

(I swear I made absolutely no attempt to make the math come out to the same estimates.)