past simple or present perfect

This is an excerpt from an official Cambridge exams preparation book:

"Mike: Thank you. Lots of people ........(help) us last year and we want to thank everyone."

The task is to fill in the correct verb form of "help". Present perfect or past simple can be chosen. The correct answer is "have helped" according to the key. I disagree. "Last year" is a finished period of time in the past, so the past simple should be used. Am I wrong?

I think the key is wrong because we can not use the present perfect with the adverbials such as last year unless the period is connected to the present moment.

we say:

1.lots of people helped us last year.

2.Lots of people have helped us since last year.

The author should have thought that present perfect was correct because the sentence ends with "we want to thank every one " which is in the present tense.

All the standard grammar books say that simple past must be used in the example.

We can not rely the study materials all the time.

I hope the native speakers agree with me.

You are right.

I here with attach two links too

When to use “in the last year”, “last year” and “in the past year”?

https://www.grammar.cl/Notes/For_Since.htm