Does this dictionary confuse "start at" with "startled at"? 我觉得是词典搞错了， 应该是 startle 才有”吃惊“的意思吧？
What you seem to be missing is that one of the meanings of the English word "start" is roughly synonymous with "startle" (in both its transitive and intransitive forms). From the Merriam-Webster dictionary:

Definition of start
intransitive verb
1 a : to move suddenly and violently, spring: *started* angrily to his feet
b : to react with a sudden brief involuntary movement: *started* when a shot rang out
[...]

transitive verb
1 : to cause to leave a place of concealment, flush: *start* a rabbit


Accordingly there does not seem to be any mistake in this dictionary, nor is this really a question about the Chinese language.