Pretty strange to read defensiveness from a 3rd party... I'm not sure why it makes sense to go to bat for an exec who was cold to someone at a trade show (which is not justifiable or productive, even if they perceive bias).
Why not review any/all of their products? On Sol-Ark's website, they currently show a 5k, an 8k, a 12k and a 15k inverter for the US market. My understanding is that they all have very similar interfaces/features (which is not a bad thing at all.) I believe the 15k inverter, which has a review, is the only one that came out recently. They also have DC optimizers, which seem to be outside of the scope of the channel. Its not like they've got a whole line of products that have been ignored. They've got batteries coming out. I imagine they'll get a review if it is feasible.
EG4 has released more products in the time that the channel has been active. There is simply more to review from them. There's nothing wrong with Sol-Ark staying focused primarily on inverters, but on a channel that seems to focus on batteries and inverters, one new product every couple years isn't going to get you a ton of screen time. That's just how it is. Doing a bunch of videos on the same sol-ark products in order to appease the executives of that company would indicate more bias than reviewing new products from another company as they come out.
One of the benefit of releasing more products is that your name will tend to show up in the reviewer space organically more often.