I saw that discussion, theory goes Orthworm became what the convergent Gyarados line would be? I think it's due to the internal names referencing land / hills so we would've had a Ground type Gyarados as a counterpart to the Wiglett / Wugtrio line, plus Earth Eater (Orthworm's ability) being the second-to-last ability in the lists with Mycelium Might (the Toedscool line's ability) being the last one
I saw that discussion, think the theory is that it got replaced by Orthworm + the convergent Tentacruel line? The internal name for Toedscool references land / hills, so Gyarados would've been a Ground type to contrast the Wiglett line (who were Ground types that became Water types). Guess Orthworm fitted the position better so it got the Ground immunity ability and we got the Toedscool line instead - Mycelium Might was the last ability added to the lists so seems to fit.
I think Orthworm existed before Okagyarados got removed. Its internal name (MIMIZU; 'earthworm') is listed way before the paradox pokémon and the legendaries, so it had likely been designed early on in development. In my opinion, it was likely designed to be one of the Titan boss fights from the start, much like Klawf.
Toedscool and Toedscruel are among the last in the internal order of pokémon. All the pokémon from Tinkatink to Annihilape were likely designed well under way into development. Those ten pokémon were likely designed to round out the pokédex, like giving story characters signature pokémon etc. In the case of the land Magikarp / Gyarados, they were designed to be thematic counterparts to the Wiglett line.
So ultimately, I think they got reworked into the Toedscool line because the design of Okagyarados was deemed to similar to Orthworm, and may have made Orthworm seem redundant in the Paldea dex.