Alonso's take loses a lot of credibility when you see how much SPs were paid after 2019 (Cole, Strasburg, Ryu, Wheeler, etc). If owners/Manfred colluded to depress the following season's strong FA SP market, but then proceeded to spend hundreds of millions on the pitchers anyway (including the biggest pitching contract ever), that seems to pour a lot of cold water on Alonso's point.
Theo Epstein in an interview said that the three most popular outcomes for fans during a game are a triple, double and SB in that order. I think deadening the ball is as simple as Manfred wanting to reduce home runs and increase BIP, and being naive enough to think that making the HR more difficult would cause hitters to change their approach. Intentionally deadening the balls so that Carlos Correa and Trevor Story get a bit less money is tinfoil hat material.