Electronic – Does unity gain Sallen-Key filter fulfill Bessel filter approximation

Forget Sallen/Key for the moment, go back to the equivalent passive implementation of the same filter. (e.g. search "passive butterworth filter" for examples.) You'll see it involves an L and a C per 2nd-order filter section, thus it's basically an L-C resonant circuit, with resistive damping to control the resonance (the height of the knee).

Having understood this, then you can use a C in the feedback path of an amplifier to simulate the L in an L-C resonant circuit - and the Sallen-Key circuit is basically one example of this. As "FakeMoustache" comments, the gyrator is another.

Please remember that when using sim.okawa-denshi.jp it is up to you to know how to properly create the topology in question. It is ill advised to create a Sallen-Key topology with a gain greater than 2 because it tends to oscillate. In fact, the page itself even states (after typing in your numbers) that it will oscillate at a frequency of 401.949Hz. If you require more gain then please create another stage that focuses on that.

Edit: Even better, I noticed that you Q-factor and your Damping factor are both negative. That is a sure indication that you will have oscillatory behavior.

As a suggestion, please use the calculation that asks for fc, gain, and provide either the Q-factor or damping ratio. This will provide you with a more stable set-up (real op amps often require additional components for stability).