这里的 to say that 没什么特别的意思，它就是普通的不定式做主语。

不过这种句式倒是值得注意一下——To say that... is ...，意思是“说……（算）是……”。

To say that Blackpool neglected the sea is to put it mildly. 说布莱克浦疏忽了海洋，这算是委婉了（说得轻了）。

更多相关表达：

To say that he was displeased is an understatement. 说他不高兴算是重事轻说了。

To say that I didn’t like the book is an understatement. 说我不喜欢这本书，那还是客气了。

To say that you are a poor cook is to do yourself an injustice. 说你是个差劲的厨师确实冤枉你了。

To say that all motorists speed in residential areas is overstating the case. 说所有的汽车驾驶员在住宅区都超速行驶是言过其实了。

To say that the proposal has no disadvantages at all is patently untrue. 要说这项提议没有缺点显然是不正确的。

To say that her resignation was a shock would be an understatement—it caused panic. 说她辞职令人震惊只是一种保守说法——它实际上引起了恐慌。

To say that this law will not affect our profits is a nonsense. 说这一法律不会影响我们的利润，这是胡言乱语。