It’s definitely not a FU. Take into account that the author is Japanese and put the pic into context. This gesture (cf. https://files.tofugu.com/articles/japan/2014-01-30-japanese-body-language/gesture-challenge-accepted.jpg) means “I can do it”/“Challenge accepted” (source: https://www.tofugu.com/japan/japanese-body-language/).
 
After all, simply google translating the author’s reply tweet in the source reveals that “There is no malice in Twi”.

Talk shit (About books) ya get hit.

@Background Pony #5BDC  
…I dun get it

>fuck you (59)  
>safe (810671)
 
It’s a Christmas miracle.

If i have fingers…

I’ve always known this action to mean “up yours”, not “fuck you.” SoCal born and bred might have something to do with it.

This definitely looks more like like “fuck you” than “flexing”

@Firebrand96  
Nothing to be sorry of.
 
And as I was about to say before you edited the comment…
 
“XD, then of course it is”.

@runesorcerer  
En España decimos “corte de mangas” y “que te jodan” o “te lo metes en el culo”.
 
Or, for the English readers this “sleevecut” concept, which means “FU” and “Up yours”.

@bigladiesman
 
Don’t worry, that’s not rude at all. At least for me.
 
But yeah, @Firebrand96, we use that gesture in Mexico to express the “chinga tu madre” feeling, which could be translated literally to “fuck your mom”, but it usually means “go fuck yourself”.

@bigladiesman  
I was talking about the picture, actually, not your comment. Sorry for the misinterpretation.

@Firebrand96  
Nothing more far from reality, my Bomberpal. I just tried to avoid the “Spexico” trope. If Runsorcerer finds my answer rude, I apologize in advance. Not my intention by any mean. As isn’t my intention to offend Mexican readers, BTW. I was just stating where I am from.
 
This gesture is common in all latin countries, and in the most of Europe. For example, in Spain we call it by a name that roughly translates as “sleevecut” (Rarara would love tis name). In Brazil, it’s a “banana”. It’s also made in Germany and Poland, where it became notorious after a Pole pole (hahahahaaaa, redunans iz fani) vault jumper flipped like just this the Russian public that booed him during the 1980 Olympics when he made the winner jump and took the gold to Poland.

@bigladiesman  
That’s the cutest “FU” I’ve ever seen.

Looks like she’s flipping the bird.

@runesorcerer  
Spain. Catalonia to be precise: other side of the Atlantic, fella.

@bigladiesman
 
It’s that country Mexico by any chance?

Ha

How rude.

Is Twily doing the SFW version of giving the middle finger?

Up yours (in my country this gesture is the same as giving the finger).

Can we do it?