It is not difficult to infer the intent behind what they mean and answer the question, but that would require some effort.
It is much easier to pedantically critique word choice. That requires no effort.

I will help you. When an object X is "true X", that usually means the same base item X, with greater magnitude, so "true X" would be a X-leaning political philosophy with a greater magnitude. So taking left-wing policies for example, it could be paid maternity and paternity leave, worker's rights, a social safety net, yearly vacation days, and such. In this limited example, the united states has none of these policies enforced on a national level. These are policies that would be included in even center-left politics. So it could be said that the united states has no "true left" because the policies enacted most frequently represent mostly right or far-right ideology. Tax cuts for corporations, slashing of retirement funds, removal of protection for the environment, etc.

The use of this "true left" terms in american politics is especially prescient because the liberal party, if removed from the united states and placed into any other political environment, would be a right-wing party. Basically, the liberal party in the united states isn't left-leaning if we look at global standards. It is a center-right or right-wing neoliberalism at best. Thus folks in the united states often say that there is no "true-left" party.

Hope this helps.

Leftism is about being egalitarian. Everyone is equal, power is shared equally, and that equality is protected. There many very different paths and approaches to this conceptually. @stembolts@programming.dev gave a great overview of some basic specific policies below.

Rightism is about consolidation of power. Since power and wealth are inextricably linked (two sides of the same coin, as it were), the defining feature of Rightism is hoarding of wealth and entrenchment of power.

Looking at our two major parties, their platforms, their policies, and their behavior as organizations, it’s easy to tell that neither meets the definition of Leftist.