That's not a great format for readtable to try to parse -- it's got the variable names as columms and so are interpreted as data -- and the 'TreatAsMissing' works only for numeric variables for some reason I've never fully understood/yet understand the logic behind.
It takes quite a lot of munging on the import options object, but one can make it happen -- whether it's easier that way or to just read as cellstr array and edit in place is probably a tossup..
Var1 Var2 Var3 Var4 Var5 Var6 Var7
___________________ ______ ______ _____ ____ ____ ____
2021/07/20 17:32:11 81.00 80.00 10.20 0.00 0.00 0.00
2021/07/20 17:33:04 100.00 100.00 5.00 0.00 0.00 0.00
This doesn't return the variable names; that can be managed by reading one record first and parsing it for the variable names that can be used to set them programmatically.