The meaning of "stern and strong" is that the angels are committed to the assignment given to them by Allah. Besides Almighty Allah and His commandment, no any other factors out of pity and compassion affect their activities. They do not disobey Allah by rejecting or opposing His order and whatever He commands, they carry it out meticulously. It is clear from this explanation that the statement "they do not disobey Allah in what He commands them" means that the angels are committed to their assignment while the statement, "they act as they are bidden", means they accomplish their work according to instruction. Hence, the respected reader should not think the second statement is a repetition of the first statement if so claimed by commentators because it would not be correct. The first statement implies they do not abandon their assignment and the second statement means they carry out the instruction.