Quem te disse isso está errado. De acordo com o link do Código das Fronteiras Schengen:
- By way of derogation from paragraph 1:
(a) third-country nationals who do not fulfil all the conditions laid down in paragraph 1 but who hold a residence permit or a long-stay visa shall be authorised to enter the territory of the other Member States for transit purposes so that they may reach the territory of the Member State which issued the residence permit or the long-stay visa, unless their names are on the national list of alerts of the Member State whose external borders they are seeking to cross and the alert is accompanied by instructions to refuse entry or transit;
Este texto é uma exceção que permite que pessoas que de outra forma seriam inelegíveis sejam admitidas em outros Estados Schengen para serem admitidas para fins de trânsito para o estado que emite o visto de longa permanência (ou seja, tipo D).
As pessoas que são elegíveis para admissão a outros Estados Schengen se enquadram no parágrafo 1:
- For intended stays on the territory of the Member States of a duration of no more than 90 days in any 180-day period, which entails considering the 180-day period preceding each day of stay, the entry conditions for third-country nationals shall be the following:
(a) they are in possession of a valid travel document entitling the holder to cross the border satisfying the following criteria:
(i) its validity shall extend at least three months after the intended date of departure from the territory of the Member States. In a justified case of emergency, this obligation may be waived;
(ii) it shall have been issued within the previous 10 years;
(b) they are in possession of a valid visa, if required pursuant to Council Regulation (EC) No 539/2001 of 15 March 2001 listing the third countries whose nationals must be in possession of visas when crossing the external borders and those whose nationals are exempt from that requirement, except where they hold a valid residence permit or a valid long-stay visa;
(c) they justify the purpose and conditions of the intended stay, and they have sufficient means of subsistence, both for the duration of the intended stay and for the return to their country of origin or transit to a third country into which they are certain to be admitted, or are in a position to acquire such means lawfully;
(d) they are not persons for whom an alert has been issued in the SIS for the purposes of refusing entry;
(e) they are not considered to be a threat to public policy, internal security, public health or the international relations of any of the Member States, in particular where no alert has been issued in Member States' national data bases for the purposes of refusing entry on the same grounds.
Como você pode ver, nada neste artigo discute nada sobre uma regra de "porta de entrada".
É possível, no entanto, que seu visto tenha restrições que impeçam seu uso para sua viagem. Por exemplo, se o visto diz que é válido para a Espanha mais um trânsito através dos Estados Schengen, você provavelmente já esgotou o trânsito único através dos Estados Schengen, caso em que você pode ser obrigado a viajar diretamente para a Espanha.