Conditional. Not all LitRPGs have systems (though all have mechanics, they may be presented only to the reader and not the characters, or the characters may be aware of them from the outset - or never truly fully aware of them and not all stories with systems are LitRPGs.
For some examples:
1. There's a series of books credited to a David Bischoff (who may be a real author, or a publishing house name, a name used by a writing group) - called "The Gaming Magi" (The Destiny Dice, The Wraith Board and The Unicorn Gambit) that is an early version of GameLit. The characters briefly encounter the players near the end of the series but in a way that only the reader is completely sure what happened there.
2. Joel Rosenberg's "Guardians of the Flame" series has the characters literally transferred into the game world, where they know the general mechanics, but have no means of tracking them - no Systems, no Character Sheets, etc. And the person behind it all dies in the second chapter before he can explain what he did and why he did it.
3. You described Solo Leveling and most of the books of the Talen Saga (Level Up Zombie, My Werewolf System, My Dragon System, My Vampire System and at least one more book) in your OP.
So, in general, yes.