#devil fruit users

First things first, what counts as flying? When Pell is introduced, we had already met Buggy who can "fly" but his feet need to stay on the ground and Miss Valentine who can float. We've also seen Logia users turn into their element and seem to fly, which also doesn't count (perhaps cause they seem to be leaping slightly helped by becoming their element). So true flying must mean you have your whole body in the air and be self-propelled for a significant distance and period of time. To make it easier, I'm going to use "could they probably fly between two islands" as a measure for time and distance.

Second the line specifically says "ever discovered" so there is some room for the number to maybe be slightly higher then five if someone ate a fruit not publicly listed. For example, Hito Hito no Mi, Model: Nika shows some possible flying but it was purposefully hidden by the World Government, so there is no way it's included in the five.

With what true flying defined lets look at some devil fruit users and their abilities to fly. We're gonna stick to canon characters and devil fruits only, no smiles since they are artificial.

Tori Tori no Mi, Model: Falcon (user Pell) established flying power

Tori Tori no Mi, Model: Phoenix (user Marco) shown flying

Tori Tori no Mi, Model: Albatross (user Morgans) cannot fly as stated by Morgans himself.

Mushi Mushi no Mi, Model: Hornet (user Bian) shown flying

Mushi Mushi no Mi, Model: Rhinoceros Beetle (user Kabu) shown flying

Fish-Fish Fruit, Model: Azure Dragon (user Kaido) climbs clouds to move through the air as stated by Momo

Ryu Ryu no Mi, Model: Pteranodon (user King) shown flying

Uma Uma no Mi, Model: Pegasus (user Stronger) shown flying

Horo Horo no Mi (user Perona) Astral projection which looks like flight but doesn't move the user's actually body

Guru Guru no Mi (user Buffalo) shown flying

Ton Ton no Mi (user Machvise) similar to Miss Valentine, can float but not fly without using another form of propulsion

Hana Hana no Mi (user Robin) can make wings out of hands but cannot sustain flight more then a few seconds

Susu Susu no Mi (user Karasu) can turn into soot birds that fly

Ito Ito no Mi (user Doflamingo) attaches strings to clouds or other objects to mimic flying

Alright, based off this we have seven fruits which gives users the ability to fly which are likely to be "known fruits". Personally I would say that seven is not that different from five, especially if it can be argued some of the fruits not being known or Karasu and Buffalo using their fruits in such a unique way that other's wouldn't realize someone with their fruit could fly.

I think the line about there being "five known fruits to grant flying ability" line has held up in the past twenty years. You go Pell, with your unique ability!