不要以为now 和 at this time 总是指现在或说话时刻。副词now和介词短语 at this time 都可以指过去某个时刻，相当于then. 例如《韦氏高阶》副词 now 释义第四条及例句：

Now: at the time referred to in the past

Her parents arrived home the next day. Now the trouble really began.

I next met her again a few years later. She was now 30 years old and working for a law firm.

又如COCA例句：

My feelings at this time were still not as euphoric as they should have been.

Our thoughts at this time were that it was a lovely apartment but had the worst balcony and position we have ever had and the noise was horrendous even with all the windows &; doors closed.

你的句子中 at this time 就是指过去时间，等于then. 你按照then去理解，should have been 不就是很自然的事情吗？