This would have been true had Liriano been waived. But... he wasn't. He was traded, and it's simply not the same situation.
I think it's reasonable to assume that the Padres traded Liriano to the club that made them the best offer - in their eyes, given their needs.
But:
1) We don't know whether the Phillies had any interest in Liriano - whether Klentak spoke to the Padres about him or not. We will likely never know about this, and we should be careful that we don't let speculative "beliefs" about what "must" have been going on in Klentak's mind somehow grow into legend or "common knowledge," because these imaginings get repeated over and over on this board.
2) We don't know how the Padres value Seidenberger (the pitcher they got for Liriano). It's easy for us to just look at his stats and say, "He's crap; we could have done better than that!" It's easy, but it's not very realistic. Seidenberger is a young player, and what the Padres' scouts saw may have been very different from what somebody looking at his baseball-reference.com page sees. I'm a stats person - but with young players, scouting and projection matter, too.
3) We're going to differ about how valuable we think Liriano may be, given his minor league performances and his contract situation. Contract situation? Liriano has no options remaining, so he cannot be placed on a 40-man roster and optioned to AAA in April. Further, he has played in seven minor league seasons...so if he's not on a 40-man roster, he will become eligible for minor league free agency - I assume at the end of the 2016 season, but that may depend on whether he's signed a contract for 2016 or not; his original contract would have bound him to play in seven seasons, and he's done that.
Given the above, what should the Phillies have traded to get him, since "getting him" might well consist of watching him play for four weeks in Florida, and then cutting him loose? (He either makes the 25-man roster or he's gone.) CD said he'd rank him ahead of Lively or Windle; would it have been wise for the Phillies to trade a Windle or a Lively for that four weeks of spring play, and the chance (probably relatively small) that Liriano would force his way onto the 25-man roster?