Corbyn’s election strategy was to concentrate resources on increasing the Labour vote in Labour seats. Starmer’s strategy was to concentrate resources on increasing the Labour vote in Tory seats. It turns out that the latter is a better way to win elections even if ends-up with fewer total votes.
Yeah I don't really get why this is meant to be a criticism of him. A seat that you win with a 30% margin and that you win with a 10% margin are functionally identical in terms of your ability to get things done.
"concentrate resources on increasing the Labour vote in Tory seats"
Worked in the unusual case where the Tory vote had a) collapsed and b) been split. Probably somewhat risky and unwise in other circumstances and unexpected in this.
Not to mention Starmer was lucky not to lose his seat.