It was the awful O line play and play calling. You know - stuff often used to excuse Joe.
I'm going to assume you accept that excuse for Flacco given that you are using it for Lamar.
Also the reason the offense was so simplistic and easy to solve was because of Lamar not in spite of him. I'm not bashing him for it. He is slightly below average with learning an offense otherwise the playbook would've opened up.
The playbook at the end was basically "run around in the backfield about 2 seconds too long, hope defense breaks down, and throw it deep" and thanks to that got them garbage time score one. The defense carried the day got the ball back and a sick run after catch got number 2 (nice throw though on both those but it doesn't excuse 3 quarters of play that helped me understand what Jacksonville has dealt with for 5 years).
At the end of the day, the OLine sucked. Marty for all his faults also knew that which is why relying on them to be useful would end the exact same way it did in KC.
I think Lamar will be very good, but right now he (very) generously might be in mid to late 30s as far as ranking pure passing among QBs. Benefit of being a young rookie though is that he can fix his technique if he works at it.