Ive got mates who are professional welders offshore and they couldnt patch an underseal covered,rusty car floor lol. means nothing,any newbie with a mig setup and a van can set up as a mobile welder.Simple thing is,he did what you asked,not very neatly i might add,and he didnt clean the area otherwise he would have seen the perforated steel just above which you unearthed with the wirewheel,It still will pass the mot,resto welding quality is a big difference to 'enough to get an mot' welding..Keep searching with the wire wheel,im sure you'll find more rust holes and perforated steel,are you going to be angry at the welder for not plating these too?Youll know next time not to use him.Would you be complaining if he'd done all these repairs for £40?..