Tankies make liberals uncomfortable because liberals believe they are the furthest left you can go
Without trying to be combative, but that sounds like one of those tidbits which one side believes about the other, circulated only to divide. At least I don’t have the impression that it is a view with any footing amongst liberal-minded people.
Most liberals want to move further left, ideas like free college and public education, public transport, less corporate power and splitting up large corporations, even unconditional basic income, etc. are popular with the majority. Just violent revolution and authoritarianism won’t roll, after all, liberal means “live and let live.”
As a mixed-ideology lefty (maybe I fit within your definition of liberal), I’m not worried about tankies being too far left, not at all, rather, I am tempted to think of them as confused right wingers believing themselves to be “the left.”