Why doesn't the Brulosopher Quick Lager Method produce esters and other off flavors?

One of the keys is that recent DNA testing shows that many ale and lager strains are essentially the same.

It's simply because esters are produced only during growth phase.

Once yeast has consumed the oxygen this triggers the end of growth and into feeding phase. Seeing that the SG has reached 50% of TG is confirmation growth is done and feeding his well underway. So it's ok at this point to increase their metabolism with temperature. But not too warm or then you can get undesirable fusal alcohols.

The same rush fermenting can be done using a full pitch or existing yeast cake. Skipping growth phase completely. (no O2 needed)

Lager yeast is kept at low temps only to retard the metabolism. In simple terms make it so they only have energy for cell devision and feed slowly.

Given their additional experiments of testing lagers fermented entirely at warmer temperatures, it would seem the answer is because the lager yeasts being used simply do not produce estery flavors or other off flavors at temperatures suitable for non-lager yeasts (60s and 70s). It would seem the lager yeast is what makes the difference between a lager and ale. One commenter on their site made a really great point that historically lager yeasts and ale yeasts were not kept as pure strains, that's a more modern advancement that we have available. In which case the only way to get a lager originally was to ferment it at temperatures that would cause non-lager yeasts to go dormant.

As a long-time practitioner of this method, I'd recommend waiting longer than this article suggests. For me, the real benefit of the technique is an accelerated reduction of diacetyl at the end of fermentation, since by the time esters have maxed out (more on that below) pretty much all other potential off-flavors (higher alcohols, acetaldehyde, sulfur compounds) will have peaked, or otherwise will benefit from a warmer conditioning. I typically raise the temperature around 60-70% attenuation (off the top of my head), and depending on expected attenuation limit.

While it's true that ester formation is linked to yeast growth, it isn't a one-to-one kind of deal. For example, trusty old Brewing Yeast and Fermentation states:

"Several reports... have observed that ester synthesis occurs at low rates during the period of active yeast growth and then proceeds at high rates when yeast growth rate declines in mid to late fermentation. These authors claimed a correlation between the time of onset of rapid ester formation and cessation of lipid synthesis"

Further, according to this paper investigating levels of isoamyl acetate and ethyl acetate formation during non-agitated lager fermentations in the laboratory, both remained relatively low until about 100 hours into fermentation, at which point they increase rapidly for 1-3 days, depending on original gravity.

Taking into account that, realistically, most ester synthesis happens towards the end of active yeast growth, I prefer to wait until the very last minute to raise the temperature, allowing fast diacetyl reduction with little risk of ester formation. Remember, of course, that the exact timing will depend a lot on your temperature control setup and batch size, since unless you can remove the fermenting beer to room/ambient temperature to help it warm up, you'll have to rely on the exothermic heat of the fermentation itself to do this.

He mentions it in the linked article:

A few things we’ve learned over the last couple centuries of brewing is that yeast generally works slower at cooler temperatures and faster at warmer temperatures, most esters and phenolics are produced during the growth phase of fermentation, which in my experience lasts about 4-5 days for lager strains, and beer lagers faster at colder temperatures.

If you wait to raise the temp until about 50% fermentation, the yeast are well past their growth phase, where most esters are produced.

I've just started to use this method (once a few weeks ago, and again started tomorrow), but initial results are good.

At the beginning of the fermentation the yeast have access to some oxygen and/or stores of the metabolites made with oxygen. This allows them to replicate a few times, so naturally that's what they do. A byproduct of this is ester production, and other stuff (acetaldehyde, diacetyl, maybe some sulfury compounds, etc) that we usually don't like in our beer.

Later on, without oxygen, all they can do is try to fatten up for the winter: eat sugar, make alcohol, and consume other metabolites left over from the replication stage.

AFAIK ale and lager yeast both do this, but the timing is apparently different.