I mean, he's not wrong, as far as we know. That being said, there might be a world in which he is wrong about something here. There have been rumors since the tail-end of the offseason that Merc had a late change in design direction during the winter. Add to that the fact they were saying since the W14's reveal that there was already an upgrade coming down the line that would change the sidepods, most likely coming by the beginning of the European leg (Imola). Putting all of this together, it could very well be that this iteration of the W14 is the car they'd have wanted to start the season with. If this is true, it could very well be that they didn't eat as much into the budget as Horner makes it seem.
Ofc, the reverse is also true and they could very little budget left already.