Basically rehashes the argument that the initial dose produces a huge amount of carbonic acid, which accounts for the "missing" dkh, and as the pH rises over the course of the day, turns back into bicarbonate
which, of course, is not accurate chemistry by any stretch.
Add bicarbonate, all the alk is there instantly.
Some dissociates into H+ and carbonate, which is an alk neutral process. That is the pH lowering effect.
HCO3- --> H+ + CO3--
(HCO3- is 1 alk; H+ is -1 alk, carbonate is +2 alk; overall change is zero alk)
Some of that H+ combines with more bicarbonate to produce carbonic acid in an alk neutral process.
H+ + HCO3- --> H2CO3
(H+ is -1 alk, HCO3- is +1 alk; carbonic acid is zero alk; overall change is zero alk)
Result is increased carbonic acid, increased bicarbonate and increased carbonate. Alk is identical to that immediately after dosing.
System may have excess CO2 now and may blow it off, raising pH and leaving alk unchanged.
H2CO3 --> CO2 (blows off) + H2O
(no alk from any of these)
Photosynthesis is an alk neutral process that consumes CO2/carbonic acid. It cannot increase alk, but it does increase pH by consuming CO2:
6CO2 + 6H2O → C6H12O6 + 6O2
(no alk from any of these, but CO2 is removed)
The CO2 removal raises pH by removing carbonic acid/CO2.
H2CO3 --> CO2 (consumed by photosynthesis) + H2O
There is thus less H2CO3 (carbonic acid) and the equilibrium below shifts to the side consumed (by le Chatliers Principle) in an alk neutral process
H2CO3 <--> H+ + HCO3-
(carbonic acid no alk; H+ -1 alk + HCO3- +1 alk; overall zero alk change)
That last sentence in the quote, about turning carbonic acid into bicarbonate, seems to be their misunderstanding of alkalinity (at least as recounted here. Yes, photosynthesis does that conversion. No, it does not (cannot) raise alkalinity because that conversion also produces H+ (a negative alkalinity contributor).