(Responding to everyone’s at once)…
(1) Popping into the sacristy could be seen to be not reasonable for this priest. We do not know how much preparation time he needed to be properly disposed himself. We also do not know whether he had been given the faculties in that place, that time. Thirdly, we do not know if the pastor had announced there would be no confessions that weekend, and possibly even put that in the bulletin, but these parishioners or visitors had not read that.
but this we do know (and discussed on other threads)
(1) A limited Saturday-afternoon time window is actually not reasonable if that is the only time during the week that confession is heard in a parish, unless the region is extremely limited in its clergy personnel. (Rural parish, cluster parishes, etc.)
(2) A variety of confession times, especially throughout several neighboring parishes – times that are widely publicized – might have solved such a confusing incident in the sacristy. Thus, at St. Mary’s, confessions are heard Sat. afternoon’s, at St. Dominic’s, Wednesday evening before/after adoration; at St. Stephen’s, before the Friday a.m. Mass.
Many of us are lucky enough to have access to daily confession opportunities at our, or a nearby, parish. This is ideal (right before Mass). It means one need never be deprived of the opportunity, ever. But apparently not all pastors want to provide that, can provide that, for whatever reason. I wouldn’t know all the circumstances. But the point is, the local parishes and/or diocese should coordinate so that at the very least there is a regular expectation of frequent confession opportunities at a variety of times.
The availability question still doesn’t directly answer the OP’s question as to immediate service upon request, but if that priest had a legitimate, reasonable justification for saying no, there should have been somewhere else in the local area where they could have gone, and which was publicly known.
Keep in mind that it often does little good to try to ring up other parishes on a Saturday afternoon (to request an immediate sacrament). Typically, the church office does not answer the phone on a Saturday afternoon (because it’s not staffed), but only on M-F during often short hours. Some priests do answer phones on Saturday afternoon, but that is the exception in my area.