Improvised weapons and features that give you unusual weapons (such as the shadow blade spell) often raise questions about which ability modifier you use for the weapon attack. Page 194 of the Player's Handbook has the rule for such situations, reproduced below. #DnD pic.twitter.com/eZUub4TfW9

— Jeremy Crawford (@JeremyECrawford) January 26, 2018

It may be an exception.

The idea is alchemist fire’s specific exception overruled the general exception.

Alternatively, it doesn’t say it is considered thrown. It is a improvised range attack. It's not an exception. Ranged weapon attacks use Dex.

— Jeremy Crawford (@JeremyECrawford) January 26, 2018

Yeah I realized that halfway through that it never said it was a thrown attack. To further clarify, there's no such thing as a thrown attack. There are weapons that have a property called thrown. That is a property of those weapons, not part of how attacks work.

— Jeremy Crawford (@JeremyECrawford) January 26, 2018