A 90kg person can definitely get to it in 60 days. My only fitness before starting BJJ in 2015 was running (which I did regularly) and I weighed about 90kg. I was successfully hand walking down the length of the mats during our warmups in a couple months (though I was falling a lot in the first month, and was never stellar at it).
I'm also uncertain how 50kg is the weight of a normal person. If I get below 75kg I'm around 5% body fat, and I'm barely above the US male average height. Getting to 50kg would mean losing a substantial amount of muscle mass, I literally have never had enough fat and so little muscle (even when I was obese) to be able to healthily cut down to that weight. And, again, I'm only about 1cm over the US average.
If you're a regular runner, you probably are pretty lean, way leaner than an average techie, and probably leaner than most people. 90kg can be 90kg at 15% fat, or 90kg at 20% fat, or 90kg at 30% fat. Those would be very different starting points.
> I'm also uncertain how 50kg is the weight of a normal person. (...)
In this paragraph you seem to be interpreting "A normal person at 50kg" from the parent comment to mean "the average person, who weighs 50kg", when what they are saying is "a 50kg person who is neither too fit not too out of shape". A slim (but not extremely thin) 5'4" (1.62m) woman weighs around that much.
I'm also uncertain how 50kg is the weight of a normal person. If I get below 75kg I'm around 5% body fat, and I'm barely above the US male average height. Getting to 50kg would mean losing a substantial amount of muscle mass, I literally have never had enough fat and so little muscle (even when I was obese) to be able to healthily cut down to that weight. And, again, I'm only about 1cm over the US average.