It's more than that. The Side kick system allows you to play ANY creature as a sidekick...and since you can play one as a PC as well....
Which is why I bring up the Storm Giant as the PC sidekick. If I read it right, I get the stats that the Book gives for the creature, meaning I start with a 29 STR...if I read it right.
A +9 Dmg to Hit and Damage from the start is a pretty big boost...and it only goes up from there. I'd probably take that guaranteed +9 every turn over a ONCE use bonus action from an action surge between rests.
If the book allows me to play a sidekick as my PC, should I now expect equivalency? Other books allow may allow me to play a monster as my PC, but constrains or modifies the amount of the monster and PC-ifies it first. Tashas seems to say I can just play the sidekick as is and be fine.
I do think that it is more or less fine to play as is and can be more on par with PCs in groups that may not play with feats or groups that may be less optimized than others. Certain combos would far outshine them though.