I don't think that's any more true than the OP ascribing to the Founders a society that offers the same rights to rich and poor alike, which the Founders expressly did NOT do.The label "Democrat" or "Republican" are political labels, and in our history have been used for both liberal and conservative political parties. The words liberal and conservative, however, designate a political ideology and are not interchangeable, This can be very confusing to students taking an American history course for the first time.
College book stores sell booklets explaining the difference and definitions of the various political ideologies. What evidence is offered that the founders were liberal?
The Founders were universally liberal in terms of believing every white male free person should be allowed to act legally in self-interest to achieve the best economic and social outcome he could. They also explicitly said slavery was ok.
Liberal and conservative are just labels people put on something to denote their personal approval or disapproval. For example, a less progressive income tax with fewer exemptions could result in the very rich paying a higher % of income in taxes than they do now, but some would call that "conservative" to imply it isn't as fair.