On the other hand, May lost seats and an apparently unassailable majority to a very left-wing (an unpopular position in the UK) Labour leader who doesn't even have proper support from his own party, has said and done very questionable things regarding terrorism, has directly campaigned against the UK in the past, appointed wildly unpopular and incompetent people to key positions after most of the senior politicians in the party resigned rather than work for him, was savagely pilloried in the media (even more than he deserved) and was generally regarded as an unelectable liability. And May still lost her majority to him, leading the Conservative party to beat the Labour party by only 2% of the votes.
Lord Buckethead put in a better showing than either of them.
I think Tim Farron summed it up well:
Exactly it was May that allowed him to get those 30 seats - her unpopularity coupled with people's questionable morals when there is free stuff on the table.
I do agree with your assessment of corbyns past.