It was for 60 days. If they continued to take this much indefinitely it would surely cause troubles, but 60 days when starting from deep deficiency is reasonable.

I was very very low, googling my level 'risk of rickets' came up!

Even that sounds extreme. The "Vitamin D Hammer" for people extremely deficient is 50k IU just once, not even for a temporary period.

It is high, but it's not extreme. 50k IU just once is an equivalent of about 7000 IU daily for a week, which won't really move the needle much if you're seriously deficient (in fact, it's still within what's considered a safe daily dose for healthy people - you can produce more than that from sunlight alone). You can feel free to take your "hammer" weekly, no deficiency required.

When I took >5000 IU daily for three months, I only raised 25(OH) D level in my blood from 9 to 30 ng/ml, and there's no evidence of toxicity below 150 ng/ml.

Of course, when dealing with high doses you need to keep your levels in check, as absorption can differ between individuals.