Pretty much this. Even the best decks in a truly competitive environment get maybe a 55% to 60% winrate against the field. 80-90% implies more that you have a soft field for whatever reason, rather than a genuine reflection of how the deck does in a competitive metagame.
The highest ranked warlock deck on HSReplay (which gathers a lot of stats so is somewhat reliable) has a 62% winrate - though their winrate against mage is basically 50-50, which is problematic given that mage seems to clearly be the best deck in the wild meta. That warlock deck does include Reno - but isn't highlander, and is actually a Mechathun deck. Second highest warlock deck with a 59% winrate is Cubelock.
In fact, if you check out Tempostorm's Renolock deck - which has a lot of similarities to this one - it's tier 3 and rank 16th for decks. That's not to say that the deck is actually bad - it hit rank 1 legend, so it can definitely win plenty of games - just that an 80 or 90% winrate isn't an accurate representation of how the deck would perform in a varied competitive environment for the average person.