About seq2seq networks I have read that seq2seq is a network, similar to other networks types (CNN, RNN, ...). However, in my opinion it is actually an architecture for RNNs. Isn't that?

For example, when input and output lengths are not equal, LSTM is not applicable. LSTM is an architecture for RNNs. Instead another architecture is used consisting two units: encoder and decoder and they are built with RNNs.

So, is seq2seq a subset of RNN? a sibling of LSTM? Or it is a sibling of RNN and CNN?
seq2seq is a type of problem, that can be solved with LSTM, RNN etc...

Generally one thinks about sequences of data that are inter connected and these relationships in sequences should be modeled (which can be achieved with for example LSTM etc.)