Hiruko obviously wouldn't make her feelings for Takumi clear until she is completely sure that they finally escaped the loop, throughout SF it is made quite clear that she although she doesn't show it openly she is quite traumatized by the experience and is always afraid that no matter what she does she will always be trapped in the loop, no way she would openly tell Takumi that she loves him and start a relationship with him unless she is sure that she will not wake up the next day back at the beginning of the hundred days having to see how she is again almost a stranger to Takumi and he might even fall in love with another girl, the fact that she confessed her feelings for Slasher is because the situation was completely screwed up and she simply did not want both Takumi and her to spend her last moments with only bad memories.