If you google "aspirin not effective against clots afib" there are many entries. Here are just two:

Aspirin monotherapy, or using aspirin by itself, was previously an option to prevent unwanted blood clots for patients with atrial fibrillation. However, further studies have shown that aspirin alone is not effective in reducing the risk of disabling stroke and it is no longer recommended. https://drafib.com/blog/blood-thinners-for-afib#:~:text=Aspirin%20monotherapy%2C%20or%20using%20aspirin%20by%20itself%2C%20was,disabling%20stroke%20and%20it%20is%20no%20longer%20recommended.

About 40 percent of "a-fib" patients deemed at moderate to severe risk of stroke because of age or other conditions are prescribed aspirin alone rather than recommended blood thinners such as Xarelto (rivaroxaban) or warfarin, according to a new study...

Another heart specialist agreed. "Aspirin is not an anticoagulant and is not effective in preventing strokes in patients with atrial fibrillation," said Dr. Samuel Wann, a cardiologist at Columbia St. Mary's Hospital in Milwaukee. Wann is co-author of an editorial published with the study.
https://www.webmd.com/heart-disease/atrial-fibrillation/news/20160620/aspirin-often-wrongly-prescribed-for-atrial-fibrillation