That's all fine in theory, but do some shopping-around and you'll see it's simply untrue.

Personal anecdata:

2018, my employer didn't offer insurance. I signed up for a "silver" tier individual plan ($1250 deductible, good coverage) on the ACA marketplace and paid $170-something/mo. (after receiving ~$250/mo. in tax credit).

2019, my employer starts offering insurance, so I'm no longer eligible for the ACA tax credit. The only plan they offer costs me $125/mo., has a $4500 deductible, and extremely skimpy coverage. According to the literature they gave us, my employer's pitching-in another $400-something/mo.

So this group plan costs significantly more, has a much higher deductible, and covers significantly less (in my case, it's the difference between medication being covered and not).