Not a cardiologist but Praluent has the following indication in the USA:
PRALUENT is...indicated as adjunct to diet and maximally tolerated statin therapy for the treatment of adults with heterozygous familial hypercholesterolemia or clinical atherosclerotic cardiovascular disease, who require additional lowering of LDL-cholesterol (LDL-C).
Oddly, no mention of HoFH. But both are approved for ASCVD.
Not sure what you're expecting. It's hard to move plus 100 billion market cap stocks. Since Amgen is a real company with revenue, you have to base stock moves on changes in discounted future earnings. When Gilead announced Harvoni approval, it's stock went down the next three business days.
It's up to the company, Amgen, to make the announcement, not the FDA. Amgen will do whatever seems most appropriate to them. All companies handle things their own way and those ways can vary over time.