I'd take the question literally and explain the answer as such;
Ike is described as "The Strongest Hero" based on brute physical strength alone. I very much doubt that he is objectively the most powerful hero in Fire Emblem's history [Despite him being nearly unbeatable once you slap Ragnell on him], as the manner in which he wields his weapon relies heavily on brute strength. Just look at his attack animations, no matter how much Ashunera/Ashera [Something something lore] blesses a weapon, I highly doubt that it's easy to literally create shockwaves so strong and precise they can pierce a dragon's scales. The only real technique we see Ike use in combat is his whole Aether blade toss combo when the aforementioned skill triggers, otherwise, it's just relatively simple large, very telegraphed, incredibly deadly slashes and the like [Excluding shockwaves].
Furthermore, this can be evidenced by how Ike is portrayed in titles he features in. For instance, take his appearance in the Smash Bros. franchise, a very slow fighter who telegraphs his attacks in advance, but with attacks so powerful that any character in the game [Ike included] are damaged immensly from just a few swings. Once again, his attacks are relatively straightforward, with the exceptions being his dash, the whole fire eruption [Which is caused by him ramming Ragnell into the ground hard enough to break the earth's crust and make liquid flame come out], and of course, as previously mentioned, Aether. However, take note that due to his immense strength, Ike is among the game's slowest characters in both appearances.
Lastly, let's look at Ike's unique skill in FE: Heroes, Heavy Blade. Heavy Blade, contrary to what its name suggests, does not lower Ike's speed in exchange for an increase in attack, instead, it has it where if Ike's attack stat is higher than his foe's [Which it will be in most non-Effie circumstances, due to Ragnell's 16 might and his presumably high attack stat], his special cooldown is reduced by 1. Once again, rewarding Ike for using raw, brute strength, rather than speed.
In conclusion, Ike is, in all likelihood, not objectively the best hero [Unless Nintendo decides to make him Ryoma 2.0], but instead, the hero who has the most raw, physical strength.