It may be the point, but it's also the flaw. It imagines that politics is reducible to something akin to this line:
Greens - Labour - Lib Dem - Tories - UKIP
And that anyone who favours a party to the left of centre over a party to the right will also favour all parties to the left of that party over all parties to the right. But this is clearly nonsense, and to see why you only have to consider the Lib Dems in the centre of that line. If one voter favours the Lib Dems over the Tories because they liked the coalition but oppose Brexit how does it follow that they'll favour Labour over the Tories? If another voter favours the Lib Dems over Labour because they feel betrayed by Corbyn's damp squib approach to Brexit how does it follow that they'll favour UKIP over Labour? The whole thing assumes an ordering that simply doesn't exist. Even if you're going to accept the excessive oversimplification of a left-right line, people will still prefer the closest candidate. The unity of an anti-Tory "alliance" is a myth; most people simply don't vote that way and the key centrist constituency definitely don't.
Now, to be fair, the idea of a centre ground and thus a "key centrist constituency" is something of a myth but what is undeniably true is that elections are won by winning over voters who will consider voting for either Tory or Labour; these voters are unimpressed by the idea of a coalition against either. In fact, it's very likely that by presenting themselves as indistinguishable from the Greens the Labour party will reduce their vote by a bigger amount than winning Green voters in seats the Greens can't contest can possibly make up for.
The absurd nature of our pathetic excuse for a voting system means that our parties are already broad tent alliances - come on, does anyone really think Liz Kendall and Jeremy Corbyn should really be in the same party? - that are only barely sustainably broad. Pursuing an alliance means pushing these alliances past their breaking points. That might appeal to minority parties like the Greens and Lib Dems but it's hopelessly inappropriate for parties with credible aspirations of power.