The relevant statutory language is: "[t]he term "machinegun" means any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger. The term shall also include...any part designed and intended...for use in converting a weapon into a machinegun..."
You can read this the way Thomas does, as requiring one depression of the trigger to fire more than one shot to qualify, or you can read this the way the dissent does, reading "function" as the human operation of the trigger. Both readings are reasonable. However, given the additional language also capturing parts that would convert a non-machinegun into a machinegun, it seems pretty clear to me that Congress wanted to capture things like bumpstocks that might allow a weapon to operate in effect as a machinegun.
This is why hypertechnical readings of laws are fucking stupid. Does anyone seriously consider it likely, had they been able to predict the invention of bump stocks, that Congress would've intended to exempt them when they originally drafed 26 U.S.C. §5845?