Exactly right. Gender is performance. We commonly enhance this performance through clothing, the way we speak and how we refer to ourselves; this is not the only ways with which it's done though. I think it would be rude to criticise a trans man for affirming their gender by growing a beard for example. However when it comes to cis men in particular, the way they reinforce their gender is often mocked. So what if the plants some grows makes them feel more comfortable in their gender.

What someone sees as affirming their gender is entirely up to them. Plenty of men do feel more masculine doing things like not using a straw, eating certain meals, listening to certain music, ect. Women might wear clothing that emphasises their figure. Some trans men feel affirmed in their gender by growing a beard, trans women might train their voice to feel more feminine. Gender is a performance and we all constantly make decisions to reinforce it. Thinking growing plants is gender affirmation isn't a problem, it's just a reality you're not living. If someone grows those kinds of plants and it reinforces their gender identity, that's valid and more power to them.