Teach's statement is more believable considering we haven't even seen the upper bounds of the Yami Yami no no, like I said,
nobody has escaped from its gravitation.
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Teach's fruit is said to be the most dangerous in devil fruit history.
I think Teachs gravitational powers differ from Fujitora.
Fujitora so far has mostly been seen to use his gravity as a aggressive force, apart from lifting the rubble with his gravity, overwhelming the opponent or thing with a powerful force.
Here are some clear pointers that show the way Teach uses the Yami Yami's gravity is different from the way Issho uses the Zushi Zushi's.
Teach's is more like a creepy inescapable force, sucking everything and anything, like a vacuum, truly like a power of the most evil devil fruit.
He was also able to pull in living things such as marine soldiers and even second division commander ace with his powerful gravity.
We've seen Teach use his gravity to store things or people in his own sort of dark dimension and regurgitate them out. There he says he compresses them with "infinite power" suggesting that his gravity is inescapable again.
He is then able to regurgitate things crushed by his gravity out, (Yes even living things).
OT: Fujitora's gravity from what we've seen is more basic and straightforward than Teach's.