I put some numbers (1,2,3,4) in A1:A4
In B1:B4 I entered some letters (a,,b,c,d) and in C1:C4 some more letters
In D1 I entered =SUMIFS(A1:A4,B1:B4,"A",C1:C4,"D")
It gave me the correct answer; that is to say, the same value I get with
=SUMPRODUCT(--(B1:B4="A"),--(C1:C4="D"),A1:A4)
Tell us about your experiment

I have just looked at the on-line example! What rubbish!
It states: Each cell in sum_range is summed only if all of the corresponding
criteria specified are true for that cell
But in =SUMIFS(B2:B5,A2:A5,"=Julie",A2:A5,"=Hans") , in any A cell we have
either JULIE or HANS so nothing ever gets added.
IMHO it is Help that is wrong; the function works as advertised (see my
first post)