You are the most confusing person to talk to.
Yep, those 6 more catches as a career high that McKenzie has over Harty is a big separator.
Last year Harty was injured. So lets use Harty's 2021 and McKenzie's 2022 when they were both last healthy.
Harty - 36 catches, 570 yards, 15.9 yard average, 3 TD's/Starting QB's that year for the Saints (Trevor Siemian, Jamies Winston, Ian Book and Taysom Hill)
McKenzie - 42 catches, 423 yards, 10.1 yard average, 4 TD's/Starting QB's last year for the Bills (Josh Allen).
So yes, to predict that Harty can have better production than McKenzie is a fairy tale. Look at those numbers above....so lopsided in McKenzie's favor that it's not even close. The QB situation cannot be used as an excuse because the talent level and stability is the same.