Fasting isn't considered essential for non-SN overdoses the way it is for SN.
In the case of amitriptyline, fasting may slightly affect absorption speed I guess, but it doesn't drastically alter the outcome like it might with SN. TCAs are rapidly absorbed in the gastrointestinal tract, and food can delay absorption slightly but not significantly enough to change the toxic dose threshold. In fact, having food in the stomach might slow the onset of symptoms just a little.
Where fasting becomes more relevant is in methods where gastric upset is likely, or where vomiting reduces lethality as with SN. Amitriptyline causes delayed gastric emptying and reduced peristalsis, meaning the drug may sit in the stomach longer anyway. Vomiting is possible, but less common than with SN, especially in high doses.
Fasting is critical for SN to reduce vomiting and maximise absorption. It's why people prepare multiple drinks of SN, because if you throw up you are more likely to fail.