I agree it could be just a statement about mortality...but having wacky theories are more fun:
Tywin was poisoned, but not by the Red Viper.
as he was at the meeting mentioned earlier(on purpose, to place suspicion on RV) and was trying to frame the RV (vengeance for Dorne)for the poisoning. Kevan's motive? He was a second son, jealousy, a desire to be Master of The Rock..(pick one).my fav was all of the above, and because of them, he was in league with Varys, one of his "lil birds" even...
but that theory died when Varys had him killed.
Or did it? who was it really that was behind the attempt on Tyrion at Blackwater? Jaime can inherit nothing, as a knight of KG...Cersei and her line can't, only heir left would be Tyrion. Kevan's character never really showed any evidence of betrayal or disloyalty...and that may have been by design...the attempt on Tyrion HAD to come before Tywin, or it would look very suspicious, the new heir dying so quickly behind the father. Kevan may have had designs on this all along...
That was my first theory,but not my favorite:
From the moment we meet her, she is attempting to use a Lannister soldier to get proximal to Tywin, her target. She unexpectedly ends up with Tyrion, which seems to fit her plans well, until she realizes they are never in the same place at the same time...so she bides her time as best she can...in spite of herself, old feelings return for Tyrion and she nearly calls off her mission, begging Tyrion to just run away with her, leave all this (the game) behind and start anew as nobodys...then Tywin returns to KL in time for Blackwater, Tyrion is almost killed, then removed from power, then married...then accused. Unable to use Tyrion as a tool any longer, she makes her move, but must be careful with Tywin...can give him no reason to think she would protect Tyrion...cannot send word to him or risk her own death (or - she smugly enjoys letting Tyrion feel the betrayal intentionally, an added bonus for her) Finally, she gets Tywin alone and poisons him with something very slow yet very lethal (say...something that once ingested would prevent the intestines from absorbing nutrition...causing slow, miserable starvation...a side-effect would be food rotting in the bowels instead of broken down for absorption) to prolong his suffering over days or weeks...a day of suffering for every silver stag...
Then enter Tyrion to spoil that plan...and to end her own life, never knowing who she really was.
Or maybe Tywin's excrement was just THAT offensive, hence Tyrion knowing he was in the shitter. He knew that smell very well, having worked the sewers at The Rock.