A pulse oximiter measures the oxygen content of arterial blood in the peripheries. A heart attack is caused by a localised blockage of the arteries to the heart, causing reduced oxygen and nutrient delivery to an area of heart muscle. A heart attack alone would not affect peripheral oxygen content unless it was severe enough for a complication such as heart failure to develop, but by that stage it's much too late.

Indeed the vast majority of patients with an acute heart attack would have completely normal pulse oximetry findings, the only way to properly diagnose a heart attack is with an electrocardiogram (EKG) and/or blood tests to look for markers of heart muscle damage

While a low reading on one of those meters (coupled with symptoms) is likely a good sign that something's wrong, I think it's quite possible for the pulse oximeter to read normal even if you're having a heart attack. There's a saying in medicine that "time is muscle" because the longer you wait to "fix" an obstruction in a coronary artery, the more likely that region of heart tissue is to die. However, tissue death is not (usually) sudden, so even if you're experiencing a significant heart attack, the heart may be pumping sufficiently and the lungs functioning normally to give you a normal oxygen reading

(this is not medical advice)

I wouldn't count on it. During a heart attack, it's the blood flow to the heart muscle that's blocked, but the lungs are still working and oxygenating the blood. So these oxygen meters will show a normal value, since your finger is still receiving the normal amount of oxygen in the blood.

For a pulmonary embolism, on the other hand, I'd expect them to be useful. And always keep in mind that they can't detect carbon monoxide poisoning, even though the blood oxygen is low in that case.