My point is that "right to private property" is a far too vague "principle" for something which covers a huge number of often very different situations (from ownership of objects and pets, to that of other people and land), some of which have massive implications on another principle of Liberalism (which is supposedly the main one) - Liberty.
That "principle" alone can nullify everything else in that definition depending on interpretation, making the whole thing flawed because it includes such a massive trap.
I would say that the praxis of how Liberalism is implemented proves that point when it comes to, for example, Land Ownership which is invariably inheritable (which is actually stretching the definition of "right to private property") - something that is key in entrenching wealth (and hence liberty) inbalances across generations - as well as tending towards the highly individualistic "the owner of the land has supreme power over it" and away from accepting limitations to such supreme power when in the common interest (though, granted, in the US there are actually pretty weird cases which are the opposite of it, in the form of the power that Housing Associations have in the US, plus in other countries there are laws that allow Land to be forcefully purchased to, for example, make roads).
Anyways, I'm not saying that Liberalism has "failed to live up to the espoused principles", I'm saying that the combination of principles it is made up of per that definition in Wikipedia does not even add up to a coherent set of principles that can be achieved together because it contains inner contradictions - it cannot live up the espouse principles because if you increase some (most namelly "right to private property") you reduce others ("liberty") - in other words, to have more Liberty for more people you have to limit the rights of private property is various areas (the most obvious in past times was the when treating people as property - i.e. Slavery - whilst nowadays Land Ownership is key in reducing the Liberty of the many because it makes a world were most people are born landless and unable to acquire land).
To use a methaphor, you cannot claim that an assemblage of sprokets setup in such a way that some work against others, is a "machine" for making all the sprokets turn, since it will never be able to do so.