calm down there is no need for that. he was just relaying his experiences, no need to play the i level game. also he is relaying information from the PTR where azerite and essences are still enabled, where as on the beta they are switched off so the two really are not comparable.
but to settle this, lets have a look at max level on the beta
for reference my beta cha is 130, max hp = 25,440. so this is normal dungeon gear
unbuffed
holy light heals for 2764 (10.86% of hp) (cast time 2.3 seconds) [healing per cast time 1201.74] { mana cost 1600 healing per mana 1.73}
Flash heals for 1,896 (7.5% of hp) (cast time 1.4 seconds) [healing per second of cast time 1354.29] {mana cost 2,200 healing per mana = 0.86}
so around a 12% difference in healing with flash of light being quicker my much less mana efficient.
now lets look at infusion, so the infusion either buffs your slow cast holy light, or makes your fast cast flash of light cheaper.
flash of light with infusion costs 1540 mana changing its healing per point of mana to 1.23
holy light with infusion heals for 3506 (13.8% of hp) changing its healing per second of cast time to 1524 and its mana coefficient to 2.19)
do keep in mind also that both of these spells produce holy power when cast on a beacon target. so in essence you are looking at between 8-13% of a person hp with each spell and the decision you need to make with the infusion is can you wait that extra 1 second for the beefy holy light or do you need a faster flash to get the holy power rolling. from playing, using flash to burn mana for holy power is a really nice way to burst heal (particularly with holy avenger), where as we use holy light for more consistent healing.
i think the reason infusion makes little sense to you in that flash is intended as a mana burner now. and holy light is intended to be that big mana efficient heal. i can see why coming forma a patch where neither spell is really used outside of infusion and inside infusion you only ever really use flash if you are desperate for the big heal why this would put you off. my advice is to try it when the beta goes live, you will see there is a flow to the spec and it really dose work. it is just very different to what we have now
some additional information for reference
holy shock heals for 3130 (12.3% hp)
WOG heals for 3353 ( 13.2% of hp)
LOD heals for 939 ( 4.9% of hp on up to 5 targets )