That appears to be the website of an obscure one-man-band law firm. In any event, the website doesn't support the mistaken assertion which you made above.
A PGDL isn't a law degree, but it takes the place of one in enabling a holder of a non-law degree to become a solicitor or a barrister. The recently introduced SQE route to becoming a solicitor doesn't require a degree at all, but at present it appears that most people taking the SQE have law degrees, or non law degrees and PGDLs.
It is not necessary to obtain a qualification in a particular type of law in order to practise as a solicitor or barrister in that type of law.