Plants feel pain too
Plants respond to damage but have no nervous system, nociceptors or brain. And if plant suffering did count, livestock consume far more plants than people do.
Plants are not inert: they detect injury, send electrical and chemical signals through their tissues, warn neighbouring plants, and change their behaviour to defend themselves, which looks a great deal like what we call pain in animals. Our confidence that only creatures with brains can suffer may simply reflect how hard it is to recognise experience in beings built differently from us. If plants have some form of experience, then every diet causes harm, and vegans cannot claim the clean moral line between their food and everyone else's that they draw.
Plants detect and respond to injury, release volatile compounds, and conduct electrical signals. None of this requires or implies experience. Pain as the term is being used (something it is like to undergo) depends on structures plants do not possess: nociceptors, a central nervous system, and a brain integrating the signal into a felt state. A thermostat responds to temperature without feeling cold.
The argument is also self-defeating as a defence of eating animals. Farmed animals eat plants, and far more of them than a person eating plants directly would. If plant suffering carried moral weight, it would count decisively against animal agriculture, which routes most of its calories through animals at a substantial loss.
Treating two things as comparable when the differences that matter to the argument are large.
How it applies here: Equates signalling in an organism with no nervous system to pain in one with nociceptors and a brain. The two are described with the same word and are not the same kind of event.
Treating the absence of proof against a claim as proof of it, or the absence of proof for a claim as proof against it.
How it applies here: Rests on the possibility that plants might have experiences we cannot detect, and treats that uncertainty as though it established the claim rather than leaving it unsupported.