The Kotzker Rebbe asked from that famous Nimukei Yosef in Bava Kamma. Rabbi Yochanan holds that one is liable for sending out a fire because it is akin to shooting an arrow (isho mishum chitzov). The Nimukei Yosef explains that this is why one is permitted to light candles Friday afternoon even though they will be burning on Shabbos; since the candles were lit from before Shabbos, that is when he shot the arrow. According to this, why is Rabbi Yochanan stating here that he would have declared the fast on the tenth of Av if the fire started on the ninth?
The answer is that regarding Shabbos and damages, we are concerned with the action; when it occurred and how it happened. Regarding the Beis Hamikdosh being destroyed; we are not concerned with the action, rather with the result and it was burned on the tenth of Av. This is why Rabbi Yochanan said that if he were living in those days, he would have ordained the tenth of Av as the fast day. (Margaliyos HaShas)