Yet Corbyn, during peak Corbyn, was only able to scrape a better result than Gordon Brown did - universally regarded as one of the least competent PMs we've ever had.
As bad as May is (and she is fudging useless) a May campaign run by Lynton Crosby would look very different to one run by a couple of her mates.
Crosby was on the campaign -- he also orchestrated Zac Goldsmith's campaign to be Mayor of London, a complete disaster. Whatever Crosby had, it's over. All he offers is poison and that didn't work against Corbyn, else the Tories would have got a big majority instead of losing their small one.
If Corbyn was as unelectable as you think he is, the Tories would call an election tomorrow, giving Sir Lynton the go ahead, and actually form a government with a working majority that could pass legislation from the election manifesto. The won't do this because they know they would lose that election and no longer be in government.
Corbyn's Labour didn't peak at the general election, as the subsequent polling shows. They are getting more popular.