it's not semantic. you may or may not have a <p> in there somewhere, but what you're describing is a list. a list of links. which then you can call it a menu. horizontal or otherwise, that's not html's job. that's css's job.
since there are many ways to simulate a design, relying on semantics is what keeps a good markup above.
i see that you are probably looking at controlling the appearance by markup. but that's css's job. and if you think about what happens when there is no css possible or applied, or you don't like how the lists look rendered, you need to trust UAs and the default appearance. you need to worry about semantics when it comes to html and css when it comes to appearance.
a list can easily became a horizontal menu. it only needs css. though there are a few cases when css may not be possible, and may look like the default appearance it's causing you a disadvantage, you should stick to the recommendations: semantic html, css for presentation. don't try to fix it by ruining your markup.
i don't know about SEO, but accessibility is affected for sure by your wrong element in the wrong place serving a wrong purpose. how about when you look at a menu at the restaurant, and you have a continuous line of courses that you can't distinguish among.