Doctors misdiagnose acute indigestion as a heart attack even now, if the person presents only symptoms but the "attack" is past. The reverse is also true. The same applies to a panic attack. These frequently, even in the 21st century, get misdiagnosed as a "mild (or otherwise) heart attack" because the patient presents the same symptoms as a heart attack.
The doctor would need to see the patient during the "attack" to make a correct diagnosis .. and these days we have all kinds of machinery to run tests during an attack that were not around in the earlier half of last century.
If the doctor only saw the patient after they were deceased, or immediately prior to death, it would be so easy in the 1930s to misdiagnose. This is not casting aspersions on the doctor (whose handwriting is actually better than many) of the day. He would have worked with what he had.