Is it legal?

As stated by the ITTF, a racket covering should be used as it has been approved. The ITTF
does not approve a racket covering, but rather the Top-sheet (the black or red rubber).
Then manufacturers may choose a type of sponge that could hard or soft, thin or thick,
boosted or not! Moreover, any player may change the sponge and use the sponge of his
choice (this is very common in Asia). So FalcoBoosting is 100% legitimate: this technique
does not touch the approved rubber since it only boosts the sponge previously unglued.