I wouldn't welcome Celtic in the Premier League, it makes no sense, and i see these comments from them as trying to start a discussion on the topic which I don't think most people want to even have.
They gee themselves up for a game or two in the Champions League every season, if they qualify to the group stages that is, and then when they get a result or two they think they can take on the world. They are not a bad team, and it would be somewhat interesting to see how they got on for a season, but I think that a top 6 estimation is way off the mark. Plus, with Brexit and the Independence movement it would be akin to us accepting Monaco and PSG in a few years anyway.
For me Celtic need to be taking on a more active role in giving the Scottish FA a boost to make Scottish football better, more competitive, and more attractive. It has become a very minor league in Europe, and no team can even start to claim to be a European giant when playing at such a domestic standard. That would be the answer that benefits Scotland and Scottish football, not trying to sneak in to next door's garden to play because they have better equipment and a nicer lawn.
Same could arguably also be said for certain Welsh teams too, but I think that they have been involved in the English leagues now for such a long time that detangling it would be horrendous and detrimental to many. I very much dislike the term BPL that seems to be used more and more in foreign press, especially the US and Far East, when taking about our league.