SVT is a tachyarrhythmia, meaning it is too fast, but it isn't quite as risky for strokes due to clot formation in the left atrial appendage as is the case with atrial fibrillation (AF). That said, it's still an arrhythmia, so a Watchman could be installed, depending on the patient's particulars.
AF, on the other hand, is not necessarily a tachyarrhythmia. You can have AF with numerous records of HR less than 100 BPM....the limit below which a fast heart rate is not considered to be tachycardia. The problem with AF, and what makes it especially dangerous, is that the left atrium beats irregularly, chaotically, but also irregularly irregularly...if that makes sense. It is this 'irregularly irregular' rhythm, or arrhythmia, that makes pooled blood in the left atrial appendage a problem.
The Watchman does not 'treat' the arrhythmia as your wording in your last question above seems to suggest you believe might be possible. The Watchman does nothing for the heart. It takes up the internal volume of the LAA, and if it seals off with endothelial cells inside of five or six months, and if it doesn't leak (confirmed by a trans-esophageal echocardiogram, or TEE), then it will prevent any further risk of strokes since the LAA accounts for an estimated 90% of the risk of stroke when the heart is in AF.
So, yes, some patients with SVT may be offered a Watchman, but it is much more likely that an AF patient will be asked if he/she would like to try it to see if it will seal off their LAA correctly and so that they may possibly be able to stop taking an anti-coagulant.

Same here! Hang in there!