keep in mind that I said that a moderate has an advantage, not that they will always win. Outside factors can still have an impact... scandals, a candidates personality, incumbency, etc.
"Not always" is an interesting way to phrase "exactly 0% of the time", which is the frequency with which being moderate has helped a Democrat or failing to be moderate has harmed a Democrat in the last few decades. Why is the factor with the least effect on the outcome (none detectable) being treated as the biggest & most important?
its not just a case of "how moderate is the democrat" but also "how moderate is the republican"
In 2016, Trump was actually seen as the more moderate candiate
If you're looking at a poll that somehow came up with a swamp-draining, opponent-jailing, treaty-breaking, federal-agency-sabotaging, wall-building "moderate", you're looking at a poll that found a way to mangle the word's usage beyond recognition. Perhaps that has something to do with left-&-right being the wrong spectrum on which to analyze Trump (and a lot of other American politics) in the first place.
But in any case, since that's on the Republican side, all that any explanation for that would do is highlight that the dynamics aren't the same for the two parties. Republican voters are more reliable so they don't need as much convincing, and a moderate Republican is one who wants to go the same way as the extremists but just slower or not as far. Democrats have trouble getting their voters to vote, and a moderate Democrat is not one who does the same stuff as an extreme Democrat in milder form, but one who does Republican stuff (which is part of why they have the turnout problem). Also, while the Democrat version of moderate is most of those who are in elected offices, the Republican version is pretty uncommon in elected offices. So none of the same moderate-or-not analysis works for both parties.
(And even if applying the same principles to both parties were a good way to look at this, that 0% success rate on the Democrat side would still be a pretty big hill for the added stuff from the Republican side to climb. The predictions one would make based on the Republicans' moderateness or lack thereof would need a 100% success rate just to break even overall... which would mean that, even then, all you would have done is prove that it works when applied to one party but not the other, so we would need to consider moderateness only for the Republican candidate. And yet, somehow, that's the side from which we never hear anybody say "we need a moderate or we'll lose".)
Lastly, the issue is not just republicans switching sides... one of the major advantages in picking a moderate is that it is less likely to energize the opposing side's base.
Then even if the principle were sound for prior elections, it would still be irrelevant for this one because the Republican base couldn't be more energized than they are right now. (They're the side with a candidate who's actually saying what they want to hear.)
The fact is, Biden has a set of policies
Policy statements on a website nobody will read, which were probably written by somebody else instead of him for the purpose of trying to bring back an eroded base, and which don't match his record or his live observable behavior right now, mean nothing. A politician's real political message is not what gets hidden away in some obscure essay but what (s)he takes an active role in pushing out for everyone to see & hear as much as possible, so it's what we're reminded of whenever we see his/her face or hear his/her voice or read a short snippet of text from him/her, over & over again, without needing to go investigating & hunting for it. What message has Biden pushed like that? To the limited extent that there even is one at all, it's just "Trump is bad" and positive fluff equivalent to "America needs to be America again". And those are electorally useless. They inform nobody and motivate nobody.