Yes, the masses are the same:

Samsung Galaxy Tab S10 Ultra:

Length
32.64 cm
width
20.86 cm
Height
0.54 cm
weight
723 g


Samsung Galaxy Tab S9 Ultra:

Length
32.64 cm
width
20.86 cm
Height
0.55 cm
weight
737 g

The S10 is 1 tenth of a millimetre thinner, doesn't matter!

Yes, I put it under mine. Good protection, but heavy.