Dembele only came into the team at the end of October when Dier's suspension combined with Mason's injury. For the previous 15 months Poch simply hadn't trusted him, because of his poor transitioning.
Yes Poch has coached Dembele to perform at a much higher level that he has ever done before. But for all of Poch's time in charge before that Liverpool game, Mason was ahead of Dembele in the pecking order.
I accept now that Dembele is ahead. But Mason is 5 years younger and has been hampered by injuries all season (Roy has told us about the ankle injections and delaying of an operation). I think back fit again next season Mason is certainly capable of edging ahead again. While he doesn't have Dembele's strength, he's much more creative and more energetic, so there's pros and cons of both.