No because that would require a qualifier like "average first liner" or "above average top liner".
The term "first liner" is a general measure of quality. It doesn't specify if the player is an elite 1st liner, an average 1st liner, or a low end 1st liner. It just means you are of approximately comensurate value to a player who can play on a 1st line. In a 30 team league, it is a reasonable assumption that means one of the 30 best players. If you want to indicate something of a higher quality, then you need to add the appropriate qualifier.
Just like if you say someone is "over 6 feet" you can't say that means 6'6 or taller just because that is the mid-point. By the same turn a "1st liner" necessarily has to include low end ones as well as high end ones, unless you specify further.
It depends on if you're using the term as a qualitative label or a quantitative measurement though. A 1st liner is an entity with its own connotations rather than purely just a numerical measurement-- it's someone who can be relied on to carry offense on the front lines. You do not need to go out of your way to point out that qualifier in order for the assumption to be received.
If you tell someone that you think they're good enough to become a cook or a lawyer, or a doctor, or an artist, the way language generally works is that it's reasonably assumed that you're talking about a competent, respectable cook or lawyer or doctor or artist, and not one that barely meets the technical requirements, and doesn't hold up to the actual standard.
I'm not saying this is necessarily the objective, enforceable right way to look at it and the other way is wrong, but I think there is a very reasonable argument to look at it in either direction.
It isn't exactly the same thing as "over 6 feet" because that's a description of an exact measurement. The connotations between calling someone a top 30 center and a 1st line center are slightly different-- one is clear-cut and undeniable and the other isn't quite as absolute.
Particularly considering the context of what we're discussing here, it's very reasonable to assume that when a hockey fan is talking about a player projecting to be a 1st liner, they're talking about someone that they could reasonably actually want on their first line, rather than someone who would be frustrating to have on the first line. I mean, you're a hockey fan, seeing if you can get a 1st liner, in order to compete. I think it's pretty obvious that that's the actual message.
I think there's such a thing as an overly pedantic and literal when approaching discourse, and this seems like one of those cases to me.