can someone hack into my s3 with "AWS-cognito-identity-poolID" that is hard-coded?

Definition of Hack

I am not sure what hacking means in the context of your question.
I assume that you actually mean "that anyone can do something different than uploading a file" which includes deleting or accessing objects inside your bucket.

Your solution

As Ninad already mentioned above, you can use your current approach by enabling "Enable access to unauthenticated identities" [1]. You will then need to create two roles of which one is for "unauthenticated users". You could grant that role PutObject permissions to the S3 bucket. This would allow everyone who visits your page to upload objects to the S3 bucket. I think that is what you intend and it is fine from a security point of view since the IdentityPoolId is a public value (i.e. not confidential).

Another solution

I guess, you do not need to use Amazon Cognito to achieve what you want. It is probably sufficient to add a bucket policy to S3 which grants permission for PutObject to everyone.

Is this secure?

However, I would not recommend to enable direct public write access to your S3 bucket.
If someone would abuse your website by spamming your upload form, you will incure S3 charges for put operations and data storage.

It would be a better approach to send the data through Amazon CloudFront and apply a WAF with rate-based rules [2] or implement a custom rate limiting service in front of your S3 upload. This would ensure that you can react appropriately upon malicious activity.

References

[1] https://docs.aws.amazon.com/cognito/latest/developerguide/identity-pools.html
[2] https://aws.amazon.com/about-aws/whats-new/2019/08/lower-threshold-for-aws-waf-rate-based-rules/

Yes, s3 bucket is secure if you are using through "AWS-Cognito-Identity-Pool" at client side, also enable CORS which allow action only from specific domain that ensure if someone try direct upload or list bucket, will get "access-denied".