Grammarly actually suggested "a" in there but I removed it because I thought it doesn't has to be so strict (in my language it's not).
Sometimes I thought that those natives are lazy and just type whatever that is enough to understand but no need to be grammatically correct.
Whether someone is a "native" speaker of English doesn't matter. I assume this is shorthand for "high quality English that would sound natural to someone whose primary language is English." The whole "native" thing on BHW is a bit weird, but I sort of get what people mean. There probably is a correlation between "native" English speakers and natural sounding English writing, but it is definitely not perfect. I have plenty of friends whose only language is English, but whose writing is terrible.
A couple of things on writing. First, most English speakers do not care about grammar or spelling, per se. It's more a question of which grammar and spelling mistakes they make. People whose primary language is English make lots of grammar mistakes, but there are certain types of grammar mistakes they almost never make. As I mentioned above, omitting indefinite articles where they are necessary is something they would almost never do. It's not because they care about grammar, it's because it sounds very wrong.
Another example. Native English speakers will mess up possessive forms a lot (as
@Nerva pointed out someone above used "American's" instead of "Americans"). They'll put an apostrophe in the wrong place, or will omit it where it is necessary. However, a mistake I see in a lot of content on the web is omitting the 's' entirely. Something like "Best Women Athletic Clothes." Leaving out the 's' completely is not a mistake someone whose primary language is English would make, and would immediately make the writing sound low-quality to me.
Using Grammarly is good, but will miss a lot of errors. For instance, in the sentence of yours above that I quote, Grammarly only complains about the "actually" being unnecessary and that "has" is the wrong form of the verb. Those are both valid points (the second is far more important than the first). However, it fails to notice other problems that stand out. (Your English is totally fine, btw, I'm just using it as an example of things that I notice that make it sound like English is not your first language.) For instance you write "type whatever that is enough..." which sounds wrong even if it is not technically a grammar mistake. Similarly, "sometimes I thought that..." is a construction that sounds awkward.
As to whether it will matter for conversions, I think that it does. If a site has low-quality writing, I'd be less inclined to believe they have high-quality products.