To help anyone else with the same issue, I've figured out a way of doing this, I guess writing about it has helped me come up with a solution

I created this class:

public class ExactPhraseExamineValue : IExamineValue
{
  public ExactPhraseExamineValue(string phrase)
  {
    Examineness = Examineness.Escaped;
    Value = $"\"{phrase}\"";
    Level = 1;
  }

  public Examineness Examineness { get; }
  public float Level { get; }
  public string Value { get; }
}

Then do this:

query = query.And().Field("articleTag", new ExactPhraseExamineValue(tag.ToLower()));

I guess this would only work if your value in the index isn't tokenized. I do the following that takes all the tags out of the tags field and saves each one as a non-tokenized field to make it easier to search for exact matches as Lucene by default will tokenize everything:

public static void Indexer_DocumentWriting(object sender, DocumentWritingEventArgs e)
{
  var document = e.Document;
  if (e.Fields["__NodeTypeAlias"].EndsWith("Article"))
  {
    var tags = e.Fields["articleTags"].Split(",".ToCharArray(), StringSplitOptions.RemoveEmptyEntries);
    foreach (var tag in tags)
    {
      document.Add(new Field("articleTag", tag.ToLower(), Field.Store.YES, Field.Index.NOT_ANALYZED));
    }
}