Doesn't matter if McLaren do have the best chassis this year. They usually start afresh each year with a new car, make a complete hash of it then make the best improvements through the year to the point they become competitive again. The ability to go faster might actually have made their current car worse as it could affect the tyre temperatures and cause them to drop off much quicker, meaning the team would have to go slower anyway.
This is the first year with these new aero regulations. Red Bull might have a beast of an upgrade ready for next year's car that isn't compatible with this year's chassis. Even Ferrari might have something in development that could give them the leap forwards. It's taken until now for McLaren to find a front wing they were happy with the balance of, over 3/4 the way into the season.
The only reason Horner stopped calling the Renault engine a joke (reliability and power) is because his other alternative was Honda. The Renault engine still has a lot of catching up to do to be a serious challenger for Mercedes.