No, the range of telepathy with a familiar from find familiar is unaffected
You answered yourself with your quote from the flock of familiars spell
Familiars summoned by this spell can telepathically communicate with you and share their visual or auditory senses while they are within 1 mile of you.
This means that the extra ability given to the familiars from flock of familiars – the ability to communicate telepathically with them from 1 mile away, as opposed to the normal 100 feet – only applies to familiars summoned with “this spell”. In this case, the phrase “this spell” refers to flock of familiars. A familiar summoned using find familiar has been summoned with find familiar, and so you would abide by the normal restrictions for that familiar.
RAW, does this mean already having conjured a familiar through Find Familiar and then casting Flock of Familiars causes you to be worse off – during the one hour Flock of Familiars lasts – then if you had only casted Flock of Familiars?
It seems so. Upon reading the description of flock of familiars, I came across this text:
If you already have a familiar conjured by the find familiar spell or similar means, then one fewer familiars are conjured by this spell.
This means if you had 1 familiar from find familiar already and then cast flock of familiars, you’d only get 2 additional familiars. The third familiar from flock of familiars wouldn’t be summoned; you already have one, so it seems in this case you are lacking an extra familiar. This familiar that is presummoned still follows my answer above; it isn’t being summoned by flock of familiars, so in a way: Yes, you are losing out on potentially 3 long-ranged, albeit short-term, familiars.