The clot is formed in the flaccid heart
and travels in
the circulation until it becomes lodged in
an area in
which it is too large to pass, causing a blockage. Most of these cats with a thromboembolism do not recover.
It is not lucky for your cat to have cardiomyopathy, but it is lucky that it was discovered since your cat can now be placed on heart medications which will hopefully prevent embolis formation.