Indeed under that use of "Modern" you are correct and Liberalism in the first half of the 20th century was as that poster described.

However "Modern" is also often used as short for "Modern Day" which is an expression for things from the recent past and present day rather than the formal "Modern" classification in History, and what you described as "Modern" Liberalism does not match the "Modern day" practice of Liberalism.

Since I don't quite see the point of talking about Liberalism from more than half a century ago in the context of a post which is literally about what Liberalis believe (Present tense, hence about Present Day Liberalism), I assumed the previous poster meant Modern in the sense of "Modern day" as that version of Liberalism was the one that made the most sense to talk about in that context.

If the previous poster meant "Modern" in the "Modern day" sense as I claim, then they're ignorant (which is the point I was making), if however them meant it as the Historical classification of Modern as you say, then they were either being deceitful (by talking about Historical Liberalism whilst trying to pass it for present day one) or illogical (since talking about history in a context which is about present day Liberal beliefs and praxis makes no sense unless one provides the links in the chain between the talked about historical one and the present day one, which they did not provide).