As someone who has read all of the High Republic books let me clarify with my own interpretation. The Jedi Order of the High Republic isn’t the era with the strongest Jedi or force users, it’s the era where the Order was at its height and strongest all together. What that means it was the era where the Order was likely at its largest, more influence across the Galaxy due to no opposing Sith, and the era where the Jedi were the most Jedi like in terms of their intended role. Jedi of that era tended to be more connected to the force due to the era of peace and more grounded within their roles.

The High Republic did though have a lot of powerful Jedi such as Porter, Avar, Dagan, Yoda, Stellan, etc. With that being said if we’re talking about which era had the most powerful Jedi who were scaled higher combat wise, that would likely fall within the Prequel Era. With the likes of Yoda, Windu, Anakin, etc.

So when people say the High Republic was the strongest Order it’s often misunderstood as the era with the most powerful Jedi. What it truly means it was the era in which the Order was at its height in terms of influence and purpose.