A lot of people probably used concrete because they knew the mafia did it. And to be honest, it's hard to dispose of a dead body in Manhattan. No convenient woods or quarries. And you put someone in a cellar floor or a foundation, they might never be found, or at least, not found until gentrification guts the building.
In the Rollingstone article, they did mention that one mobster who later played on the Sopranos who went to that club and would often bully his way in, but there wasn't really anything to tie him to the murder itself other than that.
You don't need the mixer barrel thing. My grandad used to mix it in a wheel barrow with a spade. Just the components - cement, sand, gravel, water, and stir well. You're not talking building a sound foundation, here, you're just hiding a body. All it has to do is cover her and set.
And if you dont want to do it from scratch, there are and were many instant concrete mixes you just added water and poured. Very common DIY even in the 1950s and 1960s, people poured their own porches, prking lots, cemented pillars and fences in. Noone would have questioned someone fixing something in the cellar
