I can't agree with that assumption. In 2017 essentially both two major parties were endorsing Brexit and obviously Tories with Bojo, Davies, JRM and the like were more Brexit-y. Therefore staunch Brexiteers and swing voters went for them instead. So how does the strategy of being pro-Brexit or neutral work then? The Tories swallowed the whole UKIP vote in 2017 anyway and won the election.
As an aside it's worth mentioning that, ironically, if the Tories had a achieved a clear majority they wouldn't have gone to bed with the DUP. So on their MP base would likely have been more moderate and the influence of ERG likely diminished.
Now you're also ignoring the fact that there also swing voters who are remainers and would have coalesced around Labour had their stance been pro-Remain. Labour could have carved up a piece of the Lib Dem or SNP vote if they had offered a pro-remain alternative. For example, I generally vote Lib Dem but I'm also aware they are far from winning an election. And if in 2017 Labour had became the pro-Remain alternative to the Tories I would have gladly backed them because that issue is significant enough that it warrants casting a vote where it matters most. But they didn't, so I persisted with Lib Dems who were the only ones staunchly pro-Remain.
This is anecdotal, obviously, but so is the assumption that backing Remain would have been even worse for Labour.
Re: your first paragraph, 33% of those who voted Labour in 2015 voted to Leave the EU in 2016 so your assertion that staunch Leavers were already lost to Labour before 2017 doesn't stand up. 650,000 more Leavers voted for Labour in 2017 than did in 2015; it's unlikely that this would have happened had they campaigned for a second referendum.
It's also unlikely that all, or even most, of the 3.7m Leavers who voted Labour in 2017 would still have done so if Labour had rejected the referendum result. To take the constituencies in my previous post as an example, had 252 disgruntled Labour Leave voters switched to the Tories in Bishop Auckland, County Durham would have seen it's first Tory MP in 30 years. Darlington would have taken 1600 defections to flip. Considering that an average of about 38,000 people voted Leave in each constituency across the county (higher in the ones I'm talking about here, balanced out by much lower numbers in Durham City constituency), Bishop and Darlo would likely be Tory seats now and Sedgefield and North West Durham would have gone from safe-seats to marginals in the drop of a hat.
The nature of our electoral system means that keeping Leave voters onside was key to Labour's performance in 2017 and remains key to their chances going forward. An unwelcome by-product of FPTP is that huge proportion of Labour's Remain vote effectively counts for nothing when it comes to a General Election as it's concentrated in a relatively small number of constituencies creating a handful of huge majorities. The Labour Leave vote on the other hand is distributed more evenly and basically accounts for Labour's continued hold on it's traditional heartlands outside the big metropolitan cities. The way the two sets of voters are distributed accounts for the odd stat that, whilst only 29% of Labour's voters voted leave, 60% of Labour seats did. Labour's leave vote is disproportionately important to the party's electoral success, which is why they're very reticent to alienate Leave when there could be an election on the way.
Your argument appears to be that, had Labour gone full-Remain, the loss of the Leavers would have been compensated by attracting more Remainers, but even as it was Labour attracted 2.8 million additional Remainers between 2015 and 2017 and 57% of the Remain vote overall. I have no doubt that single-issue Remainers exist who would have voted for Labour if they'd been openly pro-Remain but the numbers suggest they don't exist in sufficient numbers in the right places to swing seats (and, as above, the way they're concentrated makes them less 'useful' in a general election anyway). There were 35 marginals (less than 5% in it) in which Labour came second to the Tories in 2017; looking at vote shares it's clear that even if everyone who voted Lib Dem/Green (the 100% Remain parties) voted Labour they'd still not have won all the marginals, never mind the deeper Tory seats Labour have to win to be looking at a majority. In a fantasy-land 2017 GE scenario where Remain voters had come out of the woodwork to back a pro-Remain Labour and win the marginals, the Lib Dems stuck together elsewhere to win their seats,and Labour didn't lose a single Leave voter, the Tories would still be the largest party by 30 odd seats and a Labour-SNP-Lib Dem coalition would have a majority of 5.