In Soil, below 6.5 PH you won't uptake ant Mag OR Cal. In Hydro, above 5.8 or below and you lose one of the two and sacrifice others. That's the only reason i was recommending Cal-Mag!
A calmag product shouldn't be different in that regard. When you supply mg by itself (to treat an mg def, or ca to treat a ca def) it's subject to the same availability chart.
The only benefit I see of a calmag product is ease of use.
Another benefit of calmag is when a ca def arises and the grower hasn't taken steps to treat it individually. Mg is easy to find. Just go to the grocery store's health & beauty section for epsom salt. But, when you need ca, it's not clear what to do. You can get calcium carbonate from crushed up antacid tablets. But, that's slow acting (probably only for soil, but still slow). Calcium nitrate is fast acting, but not as easy to find in a hurry. It also supplies N which I'd prefer not doing because there's another choice in soil:
For ca def I use
dissolved eggshell. That's calcium carbonate with the carbon part removed, the time consuming breakdown performed in advance. I prefer this over a "calmag" product because it treats a ca def more specifically. But, you have to start making it a month in advance.
I agree with you that using a calmag product is more common. For that reason your advice isn't necessarily bad or "off." It's just that a person can treat deficiencies more specifically and with less cost. Epsom salt isn't any harder to use than "calmag." But, admittedly, treating a ca def is.