Honestly I could have sworn it was said to be close to 100 years before; but I could be mis-remembering,
I suppose it's not like we have a solid idea of exactly when and why the High Republic era came to an end. Such thresholds tend to be fairly arbitrary even in real history. Fifty years pre-TPM puts it right around the time Dooku & Sifo Dias would have been padawans, and IIRC right when Sifo Dias' prophetic visions start. Given that this show seems to deal with the Sith infiltrating the Jedi; perhaps for the first time getting a purchase on some real power and leverage within the Republic, these might not be mere coincidences. Indeed this is also roughly around when Palpatine was born, or at least recruited. So if nothing else it would seem to all but guarantee an appearance by Plagueis.
Side note: One thing that I've been curious about is what would the names of the eras pre and post High Republic be called? Early Republic & Late Republic? Low Republic & Middling Republic? Either way, this all sounds like something they would get applied much later in history. Seems like the height of hubris to name a period the "high" anything while you're still in it.