This is true for MATLAB program files, but MATLAB can write to a data file whose name starts with a number or other symbol perfectly well.
fid = fopen('1234567.txt', 'wt'); for k = 1:10 fprintf(fid, '%d\t%d\n', k, k.^2); end fclose(fid); type 1234567.txt
1 1 2 4 3 9 4 16 5 25 6 36 7 49 8 64 9 81 10 100
That documentation section to which you linked appears to be incorrect. MEX-file names do need to satisfy the requirements other MATLAB program files need to satisfy (ISVARNAME) but MAT-files don't necessarily need to do so. I'm not certain about Simulink model files, though I'd guess they fall in the program file category. I'll note that to the documentation staff.