Interesting. Looks like extroversion is the best correlated (.74) which I suppose makes sense. But nearly as strong (.72) is intuition/sensing to openness to experience, which I would not have predicted. Conscientiousness to judging/perceiving is only .49 and agreeableness to thinking/feeling is only .44. MBTI has no good correlate for neuroticism.
I'm no statistician, but "is approximately as valid" looks to be overselling the situation.
0.5 correlation means it is maybe perhaps correlated given Emax view of the statistics. For normal variates you should aim for R=0.95 at least. Both MBTI and Big Five are approximately normal variates.
Also correlation is a linear operator which is not particularly sensitive to anything nonlinear.
Use a good statistical test instead.
I'm no statistician, but "is approximately as valid" looks to be overselling the situation.