Not true, before the warscroll changes as you can see on the akhelian king and the eels warscrolls still it specifically says in the weapon description that the riders attack with “x” melee weapon, and the eels/mount etc attack with “x” melee weapons. These are also considered to be AoS 1.0 warscrolls and therefore use the AoS 1.0 rules when referring to mounts. This is why people allowed the sharks/turtle/eels to reroll 1s to wound.
As the leviadon and shark no longer have these direct clarification in their weapon description, and are now up to par with AoS 2.0 warscroll wording it is 100% going to ruled against idoneth deepkin players in not letting them reroll their 1s to wound.
if you recall the pink horror icon bearer issue when tzeentch got their updated warscrolls people argued for weeks that fate dice didn’t work on the banners, the counter argument was “well that’s what we did before.” That didn’t matter, and GW had to faq it. Same thing will happen here, it’s just most people haven’t noticed the wording change yet
edit: in addition both eidolons have the up to par wording on their shoals counting as mounts. So the eidolons getting updating wording can now reroll 1s to wound on shoal attacks when they couldn’t before, but the sharks and turtle didn’t get updated wording yet can somehow still reroll it? Doesn’t make sense
