There are other technically possible reasons an administration could decide to delay a payment, but the available evidence supports that the motivation is what Trump has been accused of.
Before the memorandum was released, Republican Chris Christie publicly speculated about the content and gave out "Do me a favor though" as his example of the damning language that he believed would not be in the memorandum. That was precisely the language in the memorandum.
The administration has had plenty of time now to clear up for the public their reasons for delaying the funding and to show evidence to support that this explanation is true. Instead they have hopped through various alternative explanations, dropping them when they became unsupportable. We've gone through "They were waiting for other countries to contribute more" and landed on "We wanted them to investigate corruption". But there has been no evidence of either of those requests being made whereas the request to investigate Biden and the supposed crowdstrike server were explicit. No other requests have been made public that corroborate any other aim.
The timing of the actual release of funds correlates with the whistleblower report. No other suggested motivation gives a reasonable explanation for the end of the hold on the funds. They got caught. Period.
There has been no satisfying explanation for why several mentions of Biden were removed from the memorandum or why it was stored on a higher level security server than similar calls are held on.
Several people familiar with the content of the call have expressed their impression that it, along with other actions were intended to further the president's political aims rather than national interests. The "second hand" critique doesn't hold much water because the people who originated these reports are known. These are not coffee boys or high school students. These are not rumors.
Several statements made that apparently minimized the incriminating nature of this exchange have been shown to be false.
Trump explicitly mentions that he is sending his personal lawyer to speak with a foreign head of state about investigations into his potential political rival.
No alternative reason for withholding aid would explain the available evidence. The proposed alternatives would be expected to leave communications and evidence that is missing. The administration has had plenty of time to clear the record by showing the communications which support their alternative explanations.
Instead, their supporter's narrative has changed to "So what? He can do this if he wants"