にすぎない: no more than a stated degree or role
Limit a claim so it is not expanded into a whole explanation or a certainty. Check noun and verb connections, evaluations of quantities, and the truth of the claim itself.
Do not expand one example into the whole
これは一つの例にすぎません。 means that this is merely one example. It does not deny that it is an example; it limits treating it as representative of everything or explanatory of every case.
その話はうわさにすぎません。 calls the story just a rumor. It discourages treating it as confirmed fact, but does not logically establish that the rumor's content must be false.
Limiting a role or quantity
私は意見を述べたにすぎません。 means that the speaker merely stated an opinion. It can follow a plain-form verb, acknowledging the statement while limiting its scope rather than also treating it as a decision or command.
参加者は十人にすぎませんでした。 says that there were only ten participants. It can evaluate ten as a small number, unlike the neutral count 十人でした.
Comparing だけ and checking time
見ただけです and 見たにすぎません can both limit an action to merely looking. にすぎない often presents a degree or importance as limited in a formal explanation.
これは予定にすぎません。 means that this is only a plan. The existence of a plan differs from its actual execution, so do not read this as a completion report.
Quick check: Does 提案したにすぎません。 mean that no proposal was made?
Show the answer and why
No. A proposal was made, but nothing beyond that is asserted. Approval, a decision, or implementation cannot be assumed.