What do you mean by superior? Better peak level? Better average level? You seem to admit that you agree Djokovic has the better grass career.
Djokovic rarely plays grass warm ups, while Murray tends to. As such, Murray has won more titles. It's as simple as that. It's not as though Djokovic plays Queens every year but comes up short. I'm not saying Murray shouldn't get credit for playing and winning all these titles. All I'm saying is that it's not a simple matter of more titles = higher level.
OSG was impressive, but it's not the same difficulty as a slam. A slam is 7 Bo5 matches. The 2012 Olympics was 5 Bo3 matches followed by a Bo5 final. And Wimbledon is of course more prestigious and important, as you allude to. It's an overlooked win of Murray's but it really isn't a substitute for a slam.
The H2H is an interesting one. I have never been an advocate of making H2H the decider in this kind of comparison, as it is often skewed by context. For instance, Nadal leads the outdoor hard court H2H against Federer. But no one would ever say he's the better outdoor hard court player. It can be a useful stat, but it's only one factor. In this instance, both of the matches they played took place in the space of a single year. Around/during this time period, Djokovic also lost to Federer in 4 sets at Wimbledon and Del Potro in straights, also at the Olympics. He wasn't exactly at his grass peak, although he was prime. 2011, 14 and 15 were all better years in terms of Novak's grass level. Novak really didn't play particularly well against Murray in 2013. I'm not simply trying to play down Murray's achievements; he won those matches fair and square, and seems to match up well against Djokovic on grass. But 2012-13 Djokovic wasn't peak Djokovic, on any surface.
If we look at the grass H2H, Djokovic is 2-1 against Federer and 2-2 against Nadal. Murray is 1-2 against Federer and 0-3 against Nadal. Again, context is important. Federer and Nadal both beat Murray en-route to the Wimbledon final in 2011 and 2015 respectively, before losing in 4 to Djokovic.
Murray's great on grass, but we have good evidence Djokovic's peak is higher, plus he has double Murray's Wimbledon title haul. As such, he's the better/greater grass player in my book. A far closer comparison is Nadal and Murray