If you read more closely you'll notice they're talking about a particular kind of thrombosis[1] which is accompanied by low platelet counts. This particular symptom is usually sign of a particular autoimmune response and has an incidence of about 4-5 per million people per year.

[1]: <warning, contains images of heavy bruising> https://en.wikipedia.org/wiki/Thrombotic_thrombocytopenic_pu...