No that is 300 hps per target meaning for the durations of the ult if hitting all targets for 6 seconds that is 4people x 300HPS x 6 seconds = 7200 Healing max
Mercy is 55 HPS 60 in ult
4 people x 60 HPS x 15 seconds = 3600 Total healing
Now if you factor in her passive that will be 15 x 15 = 225 from the support passive and 60 x 4 x .25 x 15 seconds from her passive 900 healing to her self
Meaning she will at max heal 1125+3600=4725 Heal max durring her ult
Zen will heal 2475 more health in his ult then mercy.
But remember mercy can get killed durring her ult while zen cant. Meaning you can kill mercy to stop her ult earlier making it heal less.
Edited math becuase i forgot something