You could just as easily argue that the existence of the Lib Dems is the only reason they don't have a majority of 200+. We're just used to the left wing being split and the right wing not, but in reality Labour have been hamstrung like that the longest time and the Tories rarely have been.
As for the vote share, I do think its something for Starmer to be very wary of. However its not so much a reflection on him or on Labour, its more about the challenge of the political landscape. We're entering a period of politics where we have a genuine plurality of parties. The Lib Dems finally look detoxified. The Greens are benefitting from general anxiety around climate change and net zero. Reform are leaning hard into the nastier elements of populist right wing thought. All three just had their best ever election.
I don't see these parties disappearing overnight, and I don't think there's a way for any party to meaningfully eat into both Lib Dem and Reform votes at the same time. So for Labour (or the Tories) to go way to the left or right isn't going to work. FPTP already punishes inefficient vote spread, which is what did for Corbyn. He appealed strongly to a narrow range of viewpoints, and not at all to every one else, so he piled up tonnes of votes in similar constituencies and lost everywhere else.
I think what this election shows is that a) the centre ground remains the only way you can win an election under our electoral system but b) the centre ground is smaller than it ever has been. That's the challenge for Labour to navigate over the next Parliament . But I don't think it’s a reflection of him personally or Labour's policy platform. I think it’s more a reflection of the diverse and divisive viewpoints that make up the country right now.