if memory serves I read somewhere that the cap on the block size was introduced due to DDoS attacks against the network using very large blocks, is it correct?
Due to the potential for such attacks.
That's a hypothesis, and one which is especially promoted by the 'wide open blocksize' crowd.
Satoshi didn't say why he did it in the commit comments as I recall, and I've not heard any credible person say that he explained it to them as it being due to DDoS concerns or anything else.
A perfectly viable hypothesis is that he knew that some other mechanism besides gross opening the blocksize would eventually be required due to scaling issues, and he thought that this setting would give enough headroom for Bitcoin to grow to a decent size before running into problems.
I like to think that, like myself, sidechains at least occurred to Satoshi early on in his ruminations about scaling modes. Likely we'll never know though. For my part I imagined the name 'child chains', but whatever. Same diff. They are just a logical sub-set of the Bitcoin solution one way or another.